GCSE Science podcasts - Atomic structure and the periodic table

TULELA: I’m Tulela Pea, a science communicator and podcaster.

SUNAYANA: And I’m Dr Sunayana Bhargava, scientist and poet.

TULELA: And this is Bitesize Chemistry.

SUNAYANA: In this first series, we’ll be looking at atomic structure and the periodic table.

TULELA: In each episode, we’ll focus on just one topic, reminding you of the key facts and sometricks we’ve used that have helped us to remember them to help you revise for your chemistry and combined science exams.

SUNAYANA: With some real-world examples, some analogies to help us understand the ideas in different ways and we’ll round off each episode with a quick quiz and the key facts to remember.

TULELA: And we’ll be aided by our snazzy AI bot that’s got lots of attitude – his name is NNICK.

SUNAYANA: So get ready for episode one - elements, compounds and mixtures, none of which are possible without … dah, dah, dah…

BOTH: The atom!

SUNAYANA: Most of chemistry boils down to what is happening to atoms, their properties, how theycombine, their effect on each other – they're like the VIPs of the molecular world. Without atoms,there is no chemistry.

TULELA: Which might make learning it a little easier.

SUNAYANA: But far less exciting! If chemistry were a movie, atoms would be the stars of the show.

TULELA: The atom indeed! Cut up anything, cut it again and again and keep going and eventually you’ll get down the atom. So take a bar of gold, keep dividing it and eventually – atoms of gold. Take a bar of pure copper, or zinc or iron, and keep dividing and eventually – individual atoms of copper, zinc or iron.

SUNAYANA: Atoms are the smallest parts of the element that exist and pretty much all the stuff we see in the universe, otherwise known as that matter you mentioned, is made up of atoms.

TULELA: A substance made of only one type of atom is called an element. Elements of copper or zinc or iron are made only from atoms of copper or zinc or iron.

SUNAYANA: And different elements are represented by a different letter or letters – also known as the chemical symbol.

TULELA: For example, zinc is Zn, copper is Cu, iron Fe.

SUNAYANA: And that gold?

TULELA: Au.

SUNAYANA: In the next few episodes, we’ll revise how the atoms of different elements are different from each other.

TULELA: Spoiler alert – it’s about the numbers of protons.

SUNAYANA: But for now, let’s look at how early chemists looked at the different elements and tried to sort them into some kind of order by understanding the patterns in their properties in relation to each other.

TULELA: And we’re talking about the periodic table.

SUNAYANA: We absolutely are. Time for some AI lowdown. Hi NNICK, can you give us some background on the periodic table?

NNICK: Nothing I’d like better. Well, I could be out hand gliding, or playing golf or having my hair done. The periodic table is a way of arranging elements according to their properties. But why, oh why, is it called the periodic table? Well, ‘periodic’ – because it orders elements so that their properties repeat periodically, and ‘table’ because it’s a table.

TULELA: Thanks, NNICK. So about 100 elements in the periodic table, arranged according to their properties and we’ll have more to say on the periodic table in other episodes.

SUNAYANA: So, as a recap, an element is something that is made from only one type of atom, and all elements are found on the periodic table.

TULELA: But there are only about 100 elements and there are many more substances and materials that exist.

SUNAYANA: Made from all those elements which make the periodic table.

TULELA: So in order for more complex chemistry to happen, atoms combine to make molecules. This occurs when two or more atoms combine chemically, either two atoms of the same element, say two hydrogen atoms combining to produce a hydrogen molecule. Or when two or more different atoms combine. So for example, when one oxygen atom combines chemically with two hydrogen atoms to produce a molecule of water, i.e. H2O.

SUNAYANA: So that’s a molecule, two or more atoms combining chemically. What about a compound?

TULELA: So when atoms from two or more different elements combine chemically in a fixed proportion for example, a sodium atom combining with one chlorine atom, we get sodium chloride or salt. So that’s an example of a compound.

SUNAYANA: The difference between molecules and compounds can sometimes be a bit tricky as they are describing similar ideas. Think of a molecule as the smallest unit building block of a larger structure.

TULELA: A good analogy is when you build a castle with those colourful plastic building bricks which we’re not allowed to mention on the BBC. The individual bricks are atoms, and different colours are different elements. We can click two or more coloured bricks together in a specific ratio that forms a molecule. If they are a different colour say one red and one green and then we build the whole castle based on that, then the whole castle is the compound.

SUNAYANA: So compounds always contain two or more different elements that combine chemically in fixed proportions. And a molecule is two or more atoms combining chemically – and these could be the same or different atoms.

TULELA: A way to think about what combining chemically means is that those individual hydrogen atoms in the compound of water are bonded in a very particular way to the oxygen atom.

SUNAYANA: We’ll have more about bonding in other episodes.

TULELA: The property of the water – H2O - is very different to the individual property of the hydrogen or oxygen. And it’s very hard to separate the compound back to its original individual elements.

SUNAYANA: So it’s like, in coming together those elements lose their individual properties, but in combining chemically with other different elements in a specific ratio, they create a compound with a new unique identity and therefore different properties.

TULELA: Yes, exactly that! A good example is table salt – on their own you have an explosive metal, sodium, and a poisonous gas, chlorine. But together in sodium chloride – or table salt – you can make your food taste way better.

SUNAYANA: So that’s a compound. What about going the other way? Say we start with a compound and want to end up with the elements?

TULELA: Absolutely – but it’s not as simple as just pulling them apart physically, those individual elements can only be recovered from compounds by chemical reactions.

SUNAYANA: So that’s a compound. Let’s look at what a mixture is.

TULELA: This can be often confused, I know I often did. A mixture is where two or more elements or compounds are mixed together.

SUNAYANA: So they are not chemically bonded together.

TULELA: Any amount of one compound added to any amount of another….and hey, presto! A mixture. An example includes a mixture of sand and water.

SUNAYANA: Which we can separate easily using filtration.

TULELA: And we’ll have more on separating mixtures later on in the series.

SUNAYANA: Can’t wait.

TULELA: Time for a quick quiz, Sunayana. Here’s some substances, you have 5 seconds to decide whether they are an element, a compound or a mixture and why. Grab a pen and paper. Ready? Sea water.

SUNAYANA: That’s a mixture because it’s not chemically bonded and can be separated.

TULELA: Carbon dioxide.

SUNAYANA: Compound because it’s a combination of fixed proportions of carbon and oxygen atoms that are chemically bonded.

TULELA: Air.

SUNAYANA: That’s also a mixture – of nitrogen and oxygen and other gases also.

TULELA: And a bar of gold.

SUNAYANA: That’s an element. It’s only one type of atom.

TULELA: So time for a quick summary of the key facts from this episode, Sunayana. Let’s go.

SUNAYANA: An atom is the smallest part of an element that can exist.

TULELA: An element is a substance made from only one type of atom.

SUNAYANA: And there are about 100 elements in the periodic table.

TULELA: Which is how we arrange those elements into some kind of order.

SUNAYANA: A compound is two or more different elements chemically combined in fixed proportions.

TULELA: And a mixture consists of different elements or compounds that are not chemically combined. Mixtures can be separated by physical processes, such as filtration.

SUNAYANA: In episode two of this series, we’ll be looking at how our ideas of the structure of the atom developed and what our best model looks like today.

SUNAYANA: I’m Dr Sunayana Bhargava.

TULELA: And I’m Tulela Pea.

SUNAYANA: And this is Bitesize Chemistry. To hear more, search Bitesize Chemistry on BBC Sounds.

TULELA: Say bye Sunayana.

SUNAYANA: Bye Sunayana.

TULELA: Thanks for listening! See ya!

SUNAYANA: I’m Dr Sunayana Bhargava, a scientist and poet.

TULELA: And I’m Tulela Pea, a science communicator and podcaster.

SUNAYANA: And this is Bitesize Chemistry. This is the second episode in an eight-part series on atomic structure and the periodic table. In this episode, we’re going to look at the history of the atom and how that model has developed over the centuries.

TULELA: We’ll look at how this led to our understanding that atoms are made up from protons, neutrons and electrons.

SUNAYANA: And we’ll end with a quick summary of the main important facts for you to take away because life is just so darn busy these days.

TULELA: But also because that’s why we’re here after all - to help revise GCSE chemistry and combined science.

SUNAYANA: Our chat bot NNICK is with us again.

NNICK: Oh, I love Chemistry, I adore it, divine chemistry!

SUNAYANA: Before we unleash NNICK, what I like about the history of the structure of the atom is that it’s a really good example of how science works. You come up with a new idea or hypothesis, devise an experiment to test the hypothesis and if the evidence backs up your predictions then it becomes a better theory. And our understanding is a little better than it was before. And this is exactly true with how the model of the atom has developed throughout history.

TULELA: Totally! So if we begin say only 200 years ago, back then what we thought an atom looked like was actually quite basic, tiny.

SUNAYANA: Very, very tiny.

TULELA: Very, very, tiny spheres that can’t be divided. This idea was proposed by a scientist from Manchester called John Dalton in 1804.

SUNAYANA: Actually, it kinda goes all the way back to the ancient Greeks and the word atom comes from the Greek ‘atomos’ – which means uncuttable.

TULELA: Nice trivia, Sunayana! But since then, we now know that atoms are composed of electrons, protons and neutrons arranged in a particular way.

SUNAYANA: So how did we get from there to our understanding today? NNICK, can you give us a quick history of the structure of the atom?

NNICK: The history of ideas about atoms. The most sensible, mature and adult way to discuss the history of ideas about atoms is through the medium of song.

SONG
Mr John Dalton imagined that atoms
Were miniature spheres that you cannot divide
And then JJ Thomson, who probably liked snacking
Described them as a plum pudding with electrons inside

Rutherford, who was Ernest, suggested a nucleus
Which no one had ever considered before
And around that were shells filled with orbiting electrons
According to that fascinating fellow Niels Bohr

Experiments suggested the existence of protons
Which contribute to the nucleus in a positive way
Add to those the neutrons discovered by Chadwick
And that's the atomic model which we still use today, OK!

SUNAYANA: Thanks, NNICK! Lots to unpack there but sounds like there are some key moments in this history of the atom that we should explore a little. And each one has progressed our understanding. First up, JJ Thomson. From his experiments, he concluded that atoms weren’t solid spheres and he proposed that they looked more like a plum pudding.

TULELA: Who even eats plum pudding these days?

SUNAYANA: Sounds like the kind of dessert they’d have had in 1904 when JJ Thomson came up with the idea. His plum pudding model could be thought of as a positively charged dough spread out evenly in which negatively charged electrons…

TULELA: …the plums…

SUNAYANA: …were embedded. Quite simple, but an advance from the solid spheres model.

TULELA: Anyway, JJ Thomson’s plum pudding model of the atom didn’t last very long because only about a decade later, along comes Ernest Rutherford who tested Thomson’s theory and proved that the plum pudding was way past its sell-by date. And he did this by showing that the positive charge in the atom wasn’t spread out evenly and was in fact concentrated in the centre – the nucleus – where most of the mass of the atom is. And his evidence came from firing positively charged alpha-particles at a very thin sheet of gold foil.

SUNAYANA: Which I shall now demonstrate in the Bitesize studio purely by the magic of sound effects! If Thomson’s plum pudding model was correct then when I fire some alpha particles at the gold foil…

TULELA: Whoa! Careful with that sound effect, Sunayana!

SUNAYANA: Because the positive charges in the gold atoms were thought to be evenly spread out, all the alpha-particle bullets would simply pass straight through – or deflected just a tiny amount if they travelled close to an electron in the plum pudding.

But what they found is that although, yes, most of the alpha particle bullets did indeed pass straight through the foil undeflected, that a few alpha particles were deflected by extreme angles or even reflected backwards as if ricocheting from something.

TULELA: Whoa! Be careful!

SUNAYANA: And this could not have happened with the Thomson plum pudding model.

TULELA: Nice shooting, Sunayana. So, from this experiment, the new model of the atom was now one of mostly empty space where a positively charged mass or nucleus is concentrated at the centre and around this are the electrons. End of story?

SUNAYANA: Not quite yet! Because Rutherford’s model only answered some questions but not all. It didn’t tell us anything more about the electrons. To resolve those questions, we had to wait until Niels Bohr. We had to wait ‘til he proposed a new development of the previous model of the atom. In this, the electrons orbit in particular shells – or energy levels to give them their correct name – which are precisely fixed distance from the nucleus. Again, his experimental observations agreed with his theorical calculations, updating the science and our ideas.

TULELA: So what does that mean, orbiting electrons in shells of a precise distance?

SUNAYANA: I like to think of them as if I were a gymnast twirling a baton over my head and I could choose a different length baton. The electrons are in the ends of the baton spinning around as I dance. They can’t get any closer or further unless I change the baton size – which is like being in a different shell.

TULELA: Baton twirling and alpha particle sharp-shooter – you’d be so talented.

SUNAYANA: Thanks! But we’re not finished yet – because even Niels Bohr’s model needed refining and later experiments from Rutherford again led to the idea that the positively charged nucleus in the atom could be subdivided into a whole number of smaller particles called protons, with each proton having the same amount of positive charge.

TULELA: The end?

SUNAYANA: Not quite – because finally about 20 years after Rutherford’s nuclear model, in 1932 James Chadwick put the final (for the moment) missing pieces into our model of the atom when he proved the existence of neutrons – chargeless particles, also within the atom’s nucleus.

TULELA: And that is the model we use to visualise the atom today. A nucleus of protons and neutrons, with orbiting electrons at specific distances from the nucleus.

SUNAYANA: And if you decide to study chemistry or physics at a higher level, you’ll see how even that model becomes more exciting and extraordinary.

TULELA: And an exciting and extraordinary career in science awaits everyone – every nationality, every gender, every background.

SUNAYANA: I couldn’t agree more.

SUNAYANA: Time for a quick interactive quiz. Three questions, 5 seconds each – here goes…

TULELA: Question 1. Who came up with the plum pudding model of the atom?

SUNAYANA: Answer - JJ Thomson.

TULELA: Question 2. Who showed that this plum pudding wasn’t correct and what was the experiment that proved this?

SUNAYANA: It was Ernest Rutherford who fired alpha particles at gold foil and updated the atom with his nuclear model.

TULELA: And Question 3. How did Niels Bohr’s model update the nuclear model further?

SUNAYANA: He brought in the idea of electrons orbiting the nucleus in energy shells.

TULELA: OK, Sunayana, quick summary?

SUNAYANA: Sure thing.

TULELA: Let’s go. From the ancient Greeks until about 200 years ago, the atom was thought of as tiny solid spheres.

SUNAYANA: Then JJ Thomson comes up with his plum pudding model.

TULELA: Rutherford fires alpha particles at gold foil and shows JJ Thomson was wrong. His nuclear model is one with a positive nucleus.

SUNAYANA: Bohr’s orbiting electron shells helps modify Rutherford’s model.

TULELA: Protons are discovered by Rutherford. Neutrons are discovered by Chadwick and the current model of the atom is in place.

SUNAYANA: And all these developments are a really good example of how science progresses though theory, experiment and evidence.

TULELA: And now I’m hungry. Plum pudding, anyone?

SUNAYANA: In episode three of this series, we’ll be looking at the structure of the atom in more detail its size – its parts, charge and its mass.

I’m Dr Sunayana Bhargava.

TULELA: And I’m Tulela Pea.

SUNAYANA: To hear more, search ‘Bitesize chemistry’ on BBC Sounds.

TULELA: Say bye, Sunayana.

SUNAYANA: Bye Sunayana.

TULELA: Thanks for listening.

TULELA: I’m Tulela Pea, a science communicator and podcaster.

SUNAYANA: And I’m Dr Sunayana Bhargava, scientist and poet.

TULELA: And this is Bitesize Chemistry. This is the third episode in an eight-part series on atomic structure and the periodic table. In this episode we’re going to look at subatomic particles, atomic structure and electronic configuration.

SUNAYANA: And we’re looking inside at those protons, neutrons and electrons.

TULELA: We’ll be comparing the size and the charge of each of those subatomic family members.

SUNAYANA: Looking at the difference between atomic number and atomic mass.

TULELA: And how the electrons in an atom are arranged according to their energy.

SUNAYANA: So in the previous episode we explored how the model of the atom developed over the past couple of centuries. And how our current model is one of a central nucleus where all the mass is concentrated, composed of positively charged protons and uncharged or neutral neutrons and that this is surrounded by negatively charged electrons orbiting in shells at specific distances.

TULELA: And we know that atoms are very, very, very tiny. But just how small are they? Time to call NNICK, our own chemistry know-it-all.

SUNAYANA: Kind of a speaking AI Chatbot.

TULELA: Hi, NNICK. Can you give us an idea of the size of an atom please?

NNICK: An atom is very small. The radius of an atom is about 0.1 nanometres. Or 1 x 10 to the -10 metres. Or nought point nought nought nought nought nought nought nought nought nought one metres.

The radius of a nucleus is about 1 x 10 to the -14 metres. Or nought point nought nought nought nought nought nought nought nought nought nought nought nought nought one metres.

For comparison, the thickness of a piece of paper is 1 x 10 to the -4 metres. Or nought point nought nought … well, you get the idea.

TULELA: Thanks NNICK, yes I think we do get the idea – those numbers are really small – an atom 0.1 nano metres! And the radius of a nucleus ten thousand times smaller still. How do you even begin to get your head around that, Sunayana?

SUNAYANA: You tell me!

TULELA: OK, so how about some analogies? Analogies are a good way of explaining sometimes complex ideas in science. So how do you picture an atom to give you a sense of what it’s made of both and its size?

SUNAYANA: I like to think of an atom as a woodland. In the middle of the woodland is a small house where a family live. The house is like the nucleus of the atom and the family are the protons and neutrons inside. But far beyond the house are little fireflies darting about, and they’re the electrons.

TULELA: That’s a nice analogy.

SUNAYANA: What about you then?

TULELA: So I’m going to take you away from your woodland and move you into a football stadium. If we scale an atom up to the size of a football stadium and the electrons are tiny specks whizzing round the back row, then the nucleus would be on the centre spot and that would be the size of…

SUNAYANA: A football?

TULELA: No, a pea! This is where all the mass of the atom is concentrated. Most of the atom is empty.

SUNAYANA: And pretty much all its mass, the protons and neutrons, is in the nucleus of the atom with only a teeny tiny amount due to the electrons.

TULELA: Rather than calculating their actual mass in terms of kilograms, it’s much easier to say that both protons and neutrons have an atomic mass unit – or amu – of one.

SUNAYANA: And an electron?

TULELA: We can just say it’s very small as they hardly contribute anything to the overall mass of the atom.

TULELA: So that’s the mass of each of those particles, what about electric charge? We’ve previously talks about protons having a positive charge, electrons a negative charge and neutrons being chargeless which we call neutral.

SUNAYANA: We can also say that every proton in the atom has a charge of plus one. Every electron has a charge of minus one and neutrons have a charge of zero. So that when you add up all the protons, neutrons and electrons and find that the overall electric charge of the atom is zero that must mean that there are exactly the same number of protons as there are electrons as all those plus charges and minus charges cancel each other out.

TULELA: And conveniently, the number of protons that an element of a particular atom has is also how we define the atom’s atomic number.

SUNAYANA: I also like to think of the atomic number as the address of an element – it tells you how many protons are in the nucleus and also its position in the periodic table. That's what makes an element unique. So, if two atoms have a different atomic number, then they have a different number of protons and are different elements.

TULELA: And as an atom has no overall charge, the number of protons is equal to the number of electrons in that atom.

SUNAYANA: For example sodium has 11 protons, and therefore 11 electrons – to ensure it has no overall charge and remains neutral. And so, its atomic number is 11. And that number is written at the bottom of the element’s chemical symbol, in this case sodium is Na.

TULELA: What about those neutrons in the sodium nucleus? Where do they come in?

SUNAYANA: If we add together the number of protons and neutrons, we get the mass number. In the case of sodium there are 11 protons and 12 neutrons making a mass number of 23 – which we write at the top of the element’s chemical symbol.

TULELA: So, atomic number – is just the number of protons which is like the address of the element. Mass number – the number of protons and neutrons added together which is all the stuff inside the house at that address. So all we need to know about what is in any atom is found by looking at the atomic number and mass number.

SUNAYANA: Spot on.

TULELA: So let’s try this on a different element. What about krypton. Mass number 84, atomic number 36. So, dear podcast listening friend, how many protons, neutrons and electrons must krypton have? You can hit the pause button if you need to work it out or you’ve got 5 seconds before Sunayana attempts it….4…3…2…1…over to you!

SUNAYANA: So krypton has 36 protons because the atomic number is 36. 84 is the mass number which is the number of protons plus neutrons. And 84 minus 36 is the 48 so there must be 48 neutrons. And remember that in an atom, there are the same number of electrons as there are protons, so again, 36 – but they don’t contribute to the mass number because they are so tiny!

TULELA: Like your fireflies buzzing around the woodland hut.

SUNAYANA: Let’s keep with those tiny electrons whizzing around the nucleus. If you remember in the last episode, we said that they are orbiting the nucleus in shells at a particular distance.

TULELA: A bit like the planets orbiting the sun in our solar system.

SUNAYANA: Sure, well those shells can hold only a specific number of electrons. And the shells further out from the nucleus contain electrons with a higher energy. The shell nearest the nucleus can contain a maximum of 2 electrons and those electrons have the lowest energy. Then the next shell orbiting further away can contain a maximum of 8 higher energy electrons. And shell 3, further out still, also a maximum of 8 electrons with even higher energy.

TULELA: And in order to keep the atom stable, the electrons take up the lowest available energy shell first. We can use this now to fully describe an atom in terms of where the electrons are arranged according to which of the energy shells they’re in.

SUNAYANA: So for example that sodium atom with 11 protons, and 12 neutrons – it must also have 11 electrons to ensure it’s neutral, and those electrons are arranged across three shells with 2 in the lowest energy shall nearest to the nucleus, 8 in the next shell and one remaining in the third shell with the highest energy.

TULELA: And this can be shown on a diagram, with circles around the nucleus to represent the shells and crosses on the shell to represent each electron.

SUNAYANA: Or simply stated that sodium has an electron arrangement of 2,8,1. After the podcast have a go at drawing the electron arrangement of fluorine which has an atomic number of 9. And you can find the diagram to compare your answer to as well as other examples on Bitesize on the web.

SUNAYANA: Time for a quick recap, Tulela?

TULELA: Yep, start us off.

SUNAYANA: Atoms are tiny. They have a central nucleus of protons and neutrons, around which are orbiting electrons.

TULELA: Protons and neutrons have an atomic mass of one. Electrons are very small in comparison.

SUNAYANA: Protons have a charge of +1. Electrons have a charge of -1 and neutrons have zero charge.

TULELA: The number of protons in an atom is called the atomic number. The number of protons is equal to the number of electrons and this is why atoms have no overall charge.

SUNAYANA: Adding the number of protons and neutrons together gives us the mass number of the element.

TULELA: And the electrons are arranged in energy shells in a specific order, occupying the lowest available shell.

TULELA: In the next episode, we’ll be looking at isotopes, how the same element can have a different number of neutrons and therefore mass number.

SUNAYANA: I’m Dr Sunayana Bhargava.

TULELA: And I’m Tulela Pea.

SUNAYANA: And this is Bitesize Chemistry. To hear more, search Bitesize Chemistry on BBC Sounds.

TULELA: Say bye Sunayana!

SUNAYANA: Bye Sunayana!

TULELA: Bye everyone.

TULELA: I’m Tulela Pea, a science communicator and podcaster.

SUNAYANA: And I’m Dr Sunayana Bhargava, scientist and poet.

TULELA: And this is Bitesize Chemistry.

SUNAYANA: This is the fourth episode in an eight-part series on atomic structure and the periodic table. We’ll be looking at what makes isotopes of an element different…

TULELA: …Neutrons.

SUNAYANA: Spoilers! And how and why isotopes are useful in everyday life.

TULELA: And how this relates to the relative atomic mass of an atom.

SUNAYANA: And NNICK, our AI with attitude will help us along the way.

TULELA: Feel free to hit pause along the way when you need to write things down and draw some diagrams, or rewind to listen again so that those key facts stick.

In the last episode, we looked at how atoms are made up from neutrons, protons and electrons.

SUNAYANA: We defined the atomic number, which is the number of protons in the atom.

TULELA: And atomic mass – the total number of protons and neutrons in the atom. And now come along isotopes. We need to know about them to work out the relative atomic mass of an element. Isotopes are defined as a different form of the same element, so same number of protons but a different number of neutrons.

SUNAYANA: And therefore, all isotopes of the same element have the same atomic number but different mass number.

TULELA: NNICK, can you give us some lowdown on isotopes please?

NNICK: The most sensible, mature and adult way to remember it is like this:

SONG
SINGER: Please welcome my dear friends the three isotopes of hydrogen! [APPLAUSE]
How many protons have you got?
ISOTOPES: (together) One.
SINGER: How many electrons have you got?
ISOTOPES: (together) One.
SINGER: How many neutrons have you got?
ISOTOPE 1: Two.
ISOTOPE 2: Nought.
ISOTOPE 3: One.

TULELA: Thanks, NNICK. So why should we care so much about isotopes? How do we use them in the real world and what good is knowing whether an element is one isotope or another?

SUNAYANA: Understanding isotopes is essential in various scientific fields, not just chemistry. In my own subject – astrophysics – one example is that different isotopes are involved in the explosion of stars called supernovas. And in archaeology, isotopes of carbon are used to estimate how long ago a once-living organism died.

TULELA: I also know that isotopes have various applications in medicine, such as in cancer treatment and medical imaging.

SUNAYANA: It’s important to remember that although isotopes of the same element have different atomic masses, they are still the same element. They all still share the same chemical properties, but they will have different physical properties, like hardness or boiling point.

TULELA: OK, so what about chlorine? That comes in two stable isotopes, chlorine-35 and chlorine-37. And I’ll give you a clue that chlorine’s atomic number is 17. Hit that pause button for the glory of working it our yourselves, in 5,4,3,2,1.

SUNAYANA: OK…atomic number 17 must mean that there are 17 protons and therefore 17 electrons. And because the mass number is 35, to work out the number of neutrons we subtract atomic number from the mass number…. 35 minus 17. 18 neutrons.

TULELA: Correct. And similarly chlorine-37. Atomic number 17, so 17 protons, 17 electrons and the number of neutrons must be 37 minus 17 equalling 20.

SUNAYANA: In the previous episode, when we talked about atomic and mass number of an atom, we said that these are usually written alongside the element’s symbol with the mass number above the symbol and the atomic number below the symbol.

TULELA: So for example, helium is He, its atomic number is 2 which is written below the symbol and its mass number is 4 which is written above the symbol.

SUNAYANA: However, the idea of isotopes makes this just a little bit more complex.

TULELA: But only a little, I promise.

SUNAYANA: Because elements have isotopes with different masses, we now need to talk about the relative atomic mass of the element, and this takes account of all the isotopes and is given by the symbol capital A with a little r by its side.

TULELA: Ahhh, that’s so sweet. It’s easy if an element has only one isotope, because then its relative atomic mass is the same as the mass number.

SUNAYANA: But if an element has more than one isotope then it’s relative atomic mass…

TULELA: …capital A tiny r…

SUNAYANA: …is the average of the mass numbers of all the different isotopes, taking into account how abundant each isotope is.

TULELA: So this might not be a whole number when you look at that element in the periodic table.

SUNAYANA: Sometimes, that number might be rounded off to one decimal place.

TULELA: So for example, chlorine has two isotopes: chlorine 35 and chlorine 37. The abundance of chlorine 35 is 75% and the abundance of chlorine 37 is 25%. In other words, in every 100 chlorine atoms, 75 atoms have a mass number of 35, and 25 atoms have a mass number of 37. So to find the relative atomic mass of chlorine.

SUNAYANA: Capital A tiny r.

TULELA: We multiply 35 by 0.75.

SUNAYANA: The 75%.

TULELA: And add that to 37 multiplied by 0.25.

SUNAYANA: The 25%, and we get 35.5 to one decimal place and that’s what you’ll see if you look for chlorine in the periodic table. So the relative atomic mass considers all the isotopes of an element. It is different to the mass number of a specific isotope.

TULELA: Just remember to take into account all the relative abundances of each isotope and average them over.

SUNAYANA: Here’s another example you can have a go at. Bromine has two stable isotopes bromine 79 and bromine 81. Bromine 79 has an abundance of around 51%.

TULELA: That means that if you take all the bromine in the universe, 51% of that will be the isotope with mass 79.

SUNAYANA: And bromine 81 isotope has an abundance of around 49%.

TULELA: 49% of all the bromine in the universe has a mass of 81.

SUNAYANA: Use that information to calculate the relative atomic mass of bromine. Hit pause now if you want to do this yourself. OK. Hey, Tulela, how is your mental arithmetic?

TULELA: I think I’ll do this on my calculator, thanks. So 51% is 0.51 and 49% is 0.49 so it’ll be… 0.51 times 79 plus 0.49 times 81 is 80.0 when we round it off to one decimal place.

SUNAYANA: Correct! If you didn’t get that answer, then don’t worry – have another go.

TULELA: Quick recap time. Let’s go.

SUNAYANA: Isotopes of the same element have the same number of protons but a different number of neutrons.

TULELA: And so a different mass number.

SUNAYANA: Isotopes of the same element have the same chemical properties but different physical ones – like boiling point.

TULELA: The relative atomic mass of an atom takes into account the abundance of each of the isotopes of the element.

SUNAYANA: And the relative atomic mass is shown by the symbol.

BOTH: Capital A tiny r.

TULELA: In the next episode in this series, we’ll be looking at physical separation processes: filtration, evaporation and crystallisation …

SUNAYANA: Across the nation?

TULELA: Across the universe, if you want. I’m Tulela Pea.

SUNAYANA: And I’m Dr Sunayana Bhargava.

TULELA: And this is Bitesize Chemistry. To hear more, search Bitesize Chemistry on BBC Sounds. Thanks for listening! Bye.

SUNAYANA: Bye!

SUNAYANA: I’m Dr Sunayana Bhargava, a scientist and poet.

TULELA: And I’m Tulela Pea, a science communicator and podcaster.

SUNAYANA: And this is Bitesize Chemistry. This is the fifth episode in an eight-part series on atomic structure and the periodic table. In this episode, we’re going to look at the three physical separation processes which are: filtration, evaporation and crystallisation.

TULELA: And when you would use one method over another.

SUNAYANA: As always, it might be handy to write some notes or diagrams along the way so hit pause when you need to. Don’t worry, we’ll still wait for you to hit play again.

TULELA: Let’s begin then with why separating substances in chemistry is so important and where in the real world we use chemistry to do this.

SUNAYANA: It’s all about purity. A pure substance in chemistry is one that is only made from one type of element or compound - like pure water which is 100% H20 only, or a pure diamond which is 100% made from carbon.

TULELA: But it’s rare that substances are 100% pure – sometimes, they have other substances that we can’t see that has been mixed into them. Like, for example, the air which is a mixture of oxygen, carbon dioxide, nitrogen and a few other gases – or sea water which is a mixture of salt and water.

SUNAYANA: Often in chemistry, we want to purify or un-mix a mixture to end up with one or more of the pure substances which made up the mixture. And in order to do so, we have a number of different physical methods which our neural networked intelligent computer knowledge bank – otherwise known as our friendly chatbot NNICK will summarise for us. Hi NNICK, can you give us a summary of physical separation processes in Chemistry?

NNICK: So we’re doing physical separation process today are we? Well I suppose so.

SONG
The names of all the methods of separation
All extremely helpfully end in '-ation'
As for example filtration, condensation, evaporation
And of course both simple and fractional distillation
But not chromatography
Which doesn't
And hence
Is an aberration.

TULELA: Thanks, NNICK. So, a few different processes there. In this episode, we’ll look at what methods to use if you want to separate a solid out of a mixture with a liquid. So grab that pen and paper to make notes.

SUNAYANA: There are two different processes that we have in our chemistry tool box, filtration and crystallisation. Both are examples of physical separation processes which don’t involve a chemical reactions and where no new substances are made. Let’s look at how they work and when you would use one or the other. First, if you have an insoluble solid that that you want to separate from a liquid.

TULELA: Insoluble means that the solid doesn’t dissolve, it is still solid – so, for example, a mixture of sand and water, and for this we use filtration to separate the sand out of the mixture.

SUNAYANA: You may have seen this in the classroom or lab, it’s a nice simple experiment where you fold a sheet of filter pater into a cone shape, pop it into a filter funnel and pour the sand-water mixture through it.

TULELA: The filter paper has tiny holes – or pores – which allows the water to pass though as these molecules are way smaller than the holes, but the sand can’t pass through the holes and is trapped. We call what passes through the filter paper the filtrate.

SUNAYANA: In this case, that would be the water.

TULELA: Right – and what’s left in the filter paper – the residue.

SUNAYANA: In this case, the sand.

TULELA: And we’re left with the water and sand and nothing else. No new substances. Sunayana, can you think of some other every day uses of filtration?

SUNAYANA: Well, how about when you make put a tea bag into boiling water. The tea bag is like the filter paper and allows the water to pass through, but doesn’t let the solid tea leaves escape. The solid tea bags are filtered out using boiling water, but the tea flavour which dissolved in the water can pass through into the cup.

That’s filtration, useful if you want to separate an insoluble solid from a liquid. What about if you have a soluble solid and liquid mixture, otherwise known as a solution?

TULELA: Soluble means that the solid has dissolved in the liquid, like salt in water. And because it’s dissolved we can’t use filtration to separate the salt out but we can use…evaporation and crystallisation.

SUNAYANA: Here’s how you would crystalise salt water to separate the salt out.

TULELA: Pour the mixture – in this case, salty water – into an evaporating dish and begin to warm the dish with a Bunsen burner. As the solution warms up, some of the water - also known as the solvent - evaporates away and so the mixture becomes more concentrated with the salt and the solid particles of salt begin to form in the dish.

SUNAYANA: At some time, we’ll begin to see crystals of salt form – this is called the point of crystallisation. And once most of the water – the solvent – has evaporated away, we can remove the heat source. The rest of the solvent will evaporate away and the dish will cool, allowing the crystals of salt to form. And as before we’re left with just what was in the mixture – nothing new has been produced.

TULELA: That’s so cool, Sunayana. I really love this experiment as you can see the crystals of the salt begin to appear before your eyes as if by magic out of the salty water mixture.

SUNAYANA: But it’s not magic, Tulela. Maybe we should say ‘as if by chemistry’. So, I gave you some real world examples of filtration. Your turn for crystallisation.

TULELA: Well, that salty water example is a really good one. In a much more scaled-up version, we can extract table salt from salty sea water or lake water. Another example is how we can separate sugar from honey – and, every time it snows, that’s another example of a crystallisation process because that’s how snowflakes are formed.

SUNAYANA: Hands up which of you lovely podcast listening friends wants a quiz? OK. Here’s three questions, five seconds each to get the correct answer – or hit pause, look it up on the BBC Bitesize webpages and no one will ever know.

TULELA: Question 1. What process can you use to separate a insoluble solid, such as sand from water?

SUNAYANA: By filtration – filter paper, funnel – sorted.

TULELA: Question 2. Once the mixture has passed through the filter paper, what is the insoluble solid which is left in the paper called?

SUNAYANA: It’s the residue.

TULELA: And Question 3. What new substances have been made when we use crystallisation to separate salt from water?

SUNAYANA: None. Remember that in physical separation there are no new substances made.

TULELA: Time for a separation summary from this episode.

SUNAYANA: There are several processes we can use in chemistry to separate – or purify – a mixture.

TULELA: We’ve looked at two to use where we have a mixture of a solid and liquid.

SUNAYANA: Filtration for an insoluble solid from a liquid.

TULELA: The remaining solid is the residue, and the liquid is the filtrate.

SUNAYANA: And use crystallisation to separate a soluble solid from a liquid.

TULELA: And watch those crystals grow as if by chemistry.

SUNAYANA: Both filtration and crystallisation are examples of physical processes which do not involve chemical reactions and no new substances are made.

TULELA: In the next episode, we’ll look at two further methods of separating mixtures which are distillation and chromatography.

SUNAYANA: I’m Dr Sunayana Bhargava.

TULELA: And I’m Tulela Pea.

SUNAYANA: To hear more, search Bitesize Chemistry on BBC Sounds. Thanks for listening.

TULELA: Bye everyone.

TULELA: I’m Tulela Pea, a science communicator and podcaster.

SUNAYANA: And I’m Dr Sunayana Bhargava, scientist and poet.

TULELA: And this is Bitesize Chemistry. In episode six of this eight-part series on atomic structure and the periodic table, we’re going to look at more physical separation processes. In the previous episode we looked at filtration and crystallisation, which we use to separate mixtures of solids and liquids.

SUNAYANA: And in this episode, we’ll be looking at separating a liquid or many different liquids out of a solution using….

TULELA: Distillation and chromatography.

SUNAYANA: As always, it might be handy to write some notes or diagrams along the way so hit pause when you need to. Don’t worry, we’ll still wait for you to hit play again.

First up, distillation. There are two types – simple distillation and fractional distillation.

TULELA: Both types rely on the fact that every pure substance has its own unique boiling point – for example, pure water boils at 100 degrees Celsius. So let’s start with simple distillation because it’s erm… simple.

SUNAYANA: If we want to separate out a liquid from a solution then we can so long as the boiling points of the components are quite different. Let’s say we have a solution of sea water, we use simple distillation. In this case, that salt in the sea water has a much, much higher boiling point than the water. Pure water boils at a 100 degrees Celsius and the salt boils at 1,465 degrees Celsius.

TULELA: And that’s important to remember – simple distillation relies on very different boiling points of the individual pure components in the solution.

In the last episode, we talked about separating salt from water by crystallisation where we were interested in collecting the salt rather than the pure water which simply evaporated into the air. Have a listen to that episode if you need a quick recap. This time, we’re interested in purifying and collecting the water itself.

SUNAYANA: In this case, the sea water is heated in a flask. As the temperature rises, pure water evaporates and is trapped in a condensing column.

TULELA: Usually a tube surrounded by a cooling jacket or cold water flow.

SUNAYANA: Where it cools and condenses back into the pure liquid water and is collected drop by drop into a beaker, leaving the salt behind in the flask. Simple distillation. Simple. But only useful if we have a solution with substances with very different boiling points. If we want to separate mixtures of liquids with similar boiling points we need a different method, which is duh-duh-duh…

TULELA: Fractional distillation.

SUNAYANA: Let’s say we have a complex mixture. In this case, we need to use a fractionating column which is cooler towards the top than the bottom. This makes sure that only the pure liquid with a boiling point lower than the temperature at the top of the column will rise to the very top of the column before condensing. The different temperatures along the fractionating column…

TULELA: Cooler at the top, hotter at the bottom…

SUNAYANA: Mean that even if more than one liquid evaporates, the ones with higher boiling points condense lower in the column so just drip back into the flask again.

TULELA: When the first liquid has been collected, we raise the temperature until the next one reaches the top – and in this way, we can collect each of the different pure liquids one at a time. Have a look over at the Bitesize website for some diagrams of both simple distillation and fractionating columns.

SUNAYANA: A really good example of the use of fractional distillation is the process that goes into oil refineries to separate crude oil into its useful parts.

TULELA: Time for an overview from our AI chat bot, NNICK. Grab a pen and paper to take notes.

SUNAYANA: Hi, NNICK. Can you give us an overview of how crude oil is separated using fractional distillation?

NNICK: Heated crude oil is put into a tall fractionating column which has a cool top and a hot bottom.The fractions leaving the column, from top to bottom, are from (HIGH VOICE) lightest to (LOW VOICE) heaviest:

(HIGHEST VOICE) Liquified petroleum gases, petrol, kerosene, diesel, heavy fuel oil, (LOWEST VOICE) and bitumen.

‘How do you remember all this NNICK?’ you ask. Well, by using cutting edge semiconductor technology. But as humans are embarrassingly bad at remembering things, you might be better off with a mnemonic like this: Lazy Penguins Keep Drinking Hot Beverage.

SONG
PENGUINS: L-L-L-Lazy

NNICK: L-L-L-Liquefied petroleum gases

PENGUINS: P-P-P-Penguins

NNICK: P-P-P-Petrol

PENGUINS: K-K-K-Keep

NNICK: K-K-K-Kerosene

PENGUINS: D-D-D-Drinking

NNICK: D-D-D-Diesel

PENGUINS: H-H-H-Hot

NNICK: H-H-H-Heavy fuel oil

PENGUINS: B-B-B-Beverage

NNICK: B-B-B-Bitumen

TULELA: Thanks, NNICK and the penguins. So let’s summarise distillation. Simple distillation – where the components have very different boiling points. And fractional distillation – where the mixture is more complex and where boiling points are not very different.

SUNAYANA: The final physical separation method that we might use in chemistry is chromatography. And this is used if we want to separate out a mixture of a soluble substance, such as food colouring or inks and identify what are the pure chemicals that it’s made from.

TULELA: Unlike distillation, which uses different boiling points to allow separation, chromatography relies on how far the separate liquids can move – and in chromatography, there are two phases involved. The stationary phase, which is a piece of chromatography paper which is similar to filter paper. And the mobile phase which is a liquid, for example water, that moves through the stationary phase.

SUNAYANA: In paper chromatography we draw a pencil line across the chromatography paper, the stationary phase, a couple of centimetres from one end and add a spot of our unknown substance onto the pencil line. We dip the bottom of the chromatography paper into the water, the mobile phase. And we’ll see that the water will start to move up the chromatography paper and when it gets to the pencil line it will carry the spot with it as it travels up the paper.

TULELA: But because the different substances in our spot have different solubilities they will travel at different rates, and so the individual chemicals will separate out forming their own unique spot at different distances from the pencil baseline on the paper.

SUNAYANA: If you do this with food colouring or inks, they will be different colours. And if you see only one spot on the filter paper, then you know you’ve had a pure substance there all along.

TULELA: A nice way to imagine chromatography to make it more memorable is to think of it as if you have a team of three sprint runners lining up at the starting line. Each has different coloured trainers on and each colour has a different stickiness to the track. Blue is very sticky so that runner will not be able to run very fast, red is medium stickiness so that runner will get further, and yellow trainers are super-whizz new ones with very little stickiness so that runner will get furthest in the same time. And once again, with distillation and chromatography we are physically separating pure chemicals without producing anything that wasn’t already there in the first place. And you can visit the BBC Bitesize website for more information on chromatography.

So, lots of info there, Sunayana, so let’s have a quiz to get those facts to stick.

SUNAYANA: Yes, please.

TULELA: Three questions, five seconds – no prizes. So grab that pen and write your answers down. Here we go. Question 1. How does the temperature in a fractionating column change from bottom to top?

SUNAYANA: It’s hotter at the bottom and cooler at the top.

TULELA: Question 2. I have a liquid mixture of two substances. One with a boiling point of 100 degrees (which must be water) and the other with a boiling point of 2000 degrees. Which method can I use to separate them?

SUNAYANA: Simple distillation is what we need, because the boiling points are very, very different.

TULELA: And question 3. In chromatography, what are the names of the two phases and give an example of each.

SUNAYANA: We have the stationary phase, for example chromatography paper, and the mobile phase, for example water.

TULELA: End of podcast summary time – let’s go.

SUNAYANA: Right, to physically separate liquids from mixtures we can use distillation and chromatography.

TULELA: Distillation make uses of the fact that pure substances in a mixture have unique and different boiling points.

SUNAYANA: We use simple distillation where the components in the mixture have very different boiling points.

TULELA: And fractional distillation for more complex mixtures where individual boiling points are not too different.

SUNAYANA: Chromatography has a mobile phase to carry an unknown solution across a stationary phase.

TULELA: On the next episode, we’ll be looking at the development of the periodic table in a little bit more detail.

SUNAYANA: I’m Dr Sunayana Bhargava.

TULELA: And I’m Tulela Pea.

SUNAYANA: And this is Bitesize Chemistry. To hear more, search Bitesize Chemistry on BBC Sounds.

TULELA: Bye then!

SUNAYANA: See ya.

SUNAYANA: I’m Dr Sunayana Bhargava, a scientist and poet.

TULELA: And I’m Tulela Pea, a science communicator and podcaster.

SUNAYANA: And this is Bitesize Chemistry. This is the seventh episode in an eight-part series on atomic structure and the periodic table. In this episode, we’re going to look at the development of the periodic table to the one we have today, how the table arranges the elements in increasing atomic number and by their chemical properties.

TULELA: And we’ll define periods and groups.

SUNAYANA: The rows and columns in the table.

TULELA: And as always, we’ll round it off with a quick quiz and summary of all the most important facts. Ready with your pen and paper to make notes?

SUNAYANA: And hit pause and rewind when you need to, to have a little more time to let those facts really sink in.

TULELA: In previous episodes, we’ve looked at the subatomic family of particles that make up the atom. The proton and neutron in the nucleus and those electrons orbiting around in their energy shells.

SUNAYANA: But even before these particles were discovered, scientists of the time tried to arrange the elements into some kind of order by their atomic weight and properties. And it took about a hundred years of refining this order before the we got to the periodic table that you see today.

TULELA: And this table is based on one made by a Russian scientist, Dimitri Mendeleev. Time for some background info. Let’s fire up our Bitesize banter buddy, NNICK. Hi, NNICK. Can you tell us more about how Mendeleev’s idea led to the modern periodic table?

NNICK: Mendeleev wrote the names of the elements onto cards and arranged them in order of the lightest to the heaviest. Then he made groups based on their physical and chemical properties. About 50 elements were known at the time, and here's the clever bit: Mendeleev left gaps in his table for elements yet to be discovered. He was even able to predict their properties. Pretty smart for a human.

TULELA: Thanks, NNICK.

SUNAYANA: So let’s just summarise Mendeleev’s table. He arranged elements in order of increasing atomic weight. The horizontal rows are called periods. The vertical columns are called groups, and elements in the same group have similar properties to each other. But, I mean, he wasn’t the first scientist to arrange the elements in a list, nor the first to arrange them with similar properties, so what makes him the so-called ‘Father of the periodic table?’

TULELA: That is because of what was not in his original table. Mendeleev’s brilliance comes from what he left out - the gaps in his table that NNICK mentioned. Unlike previous attempts from Dalton and Newlands who arranged in atomic weight and did not leave gaps, those gaps that Mendeleev left were placeholders for elements that hadn’t been discovered at the time. He’s like saying ‘Hey, I predict that one day we will discover an element with these properties which will slot nicely in this gap’ and based on where that gap is in my table, I predict what its properties will be.

SUNAYANA: That’s a great way to show how science works. Make a prediction - or hypothesis - and later, when the evidence backs that up, the science progresses.

TULELA: And that’s exactly what happened. For example, one of the gaps in Mendeleev’s table is associated with an element with an atomic weight of around 68. An element that hadn’t been discovered. But Mendeleev predicted its properties, that it would be a solid metal at room temperature and that its melting point was likely to be quite low. A few years later, the previously unknown metallic element gallium was discovered precisely with those properties and that gap was filled.

SUNAYANA: And what’s even more amazing is that some of the elements he predicted weren’t discovered until many years after he died.

TULELA: All hail Dimitri Mendeleev – the ‘Father of the periodic table’.

SUNAYANA: Yes, however although his table arranged the atoms by their atomic weight he did also swap around some elements because of their properties as that made them fit better. This insight eventually was shown to be correct when the existence of isotopes was discovered.

TULELA: Remember, isotopes of an element are different forms of the same element. So, same number of protons but different number of neutrons – have a listen to episode 4 for a quick refresh.

SUNAYANA: Not accounting for isotopes would have led to a few elements ending up in the wrong columns, like iodine and tellurium. Tellurium has a greater atomic weight than iodine but one fewer proton so would have been placed after iodine on Newland's and Dalton's tables instead of before as in the modern table. So instead of arranging by atomic weight, which is the number of protons plus neutrons, the modern periodic table arranges the elements by atomic number.

TULELA: Just the number of protons.

SUNAYANA: Let’s look a little bit more detail at the table itself. It might be useful if you have a copy of one in front of you or one on your classroom wall.

TULELA: Either way, make sure you find the one issued by your exam board if you look it up online.

SUNAYANA: You’ll see that as we go from left to right across the table, the atomic number increases by one and these horizonal rows are called periods, and the period that each element belongs to represents the number of electron shells that the atom has.

TULELA: So for example, the second row down – or period 2 – is lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine and neon and each of these elements has two electron shells.

SUNAYANA: And as we go down the vertical columns, the groups, the elements all have the same number of electrons in their outer-shell.

TULELA: So in group 1 (ignoring hydrogen for the moment) we have lithium, sodium, potassium and so on – all have one outer-shell electron.

SUNAYANA: Whereas group 7, fluorine, chlorine, bromine, iodine all have seven.

TULELA: And group 0, the far-right column with helium, neon, argon and krypton have a complete outer-shell.

SUNAYANA: Elements in the same group have similar properties. For example, in group 1 (that group with lithium, sodium etc.) are all called alkali metals with relatively low melting points and can all be cut with a knife.

TULELA: You’ll see that between group 2 and group 3 there’s a gap where there’s a block of elements with similar properties and these are called transition metals.

SUNAYANA: Time for a quick quiz. Write your answers down, here goes. I’ll give you an element and you tell me what period and group it’s in on the table and therefore how many electron shells it has and how many electrons in its outer shell. It would help if you've got a periodic table in front of you.

TULELA: Go for it.

SUNAYANA: Nitrogen.

TULELA: The answer is period 2. Group 5. Two shells. Five electrons in the outer shell.

SUNAYANA: Sodium.

TULELA: That answer is period 3, group 1. So three shells and one electron in the outer shell.

SUNAYANA: And, argon.

TULELA: That’s period 3. Group 0. So three shells, but a full complete eight electrons in the outer shell.

SUNAYANA: Nicely done!

TULELA: Thanks, Sunayana. Time for a re-cap.

SUNAYANA: The periodic table developed when the early chemists attempted to list the known elements into some kind of order.

TULELA: The early periodic table was arranged by atomic weight and the properties of the elements. Mendeleev added to this by rearranging the order of some and leaving gaps where he predicted unknown elements would be found.

SUNAYANA: His table is the basis of the one we use today, although we order by atomic number not weight.

TULELA: The rows are the different periods and defines how many electron shells there are in the atom.

SUNAYANA: The columns are the groups and define how many electrons there are in the outer shell.

TULELA: Elements in the same group have similar chemical properties. In the next episode, there’ll be more on the periodic table, specifically group 1, 7 and 0.

SUNAYANA: I’m Dr Sunayana Bhargava.

TULELA: And I’m Tulela Pea.

SUNAYANA: And this is Bitesize Chemistry. To hear more, search Bitesize Chemistry on BBC Sounds. Thanks for listening!

TULELA: I’m Tulela Pea, a science communicator and podcaster.

SUNAYANA: And I’m Dr Sunayana Bhargava, scientist and poet.

TULELA: And this is Bitesize Chemistry. In this final episode of an eight-part series on atomic structure and the periodic table, we’re going to look at three groups in periodic table – specifically group 1, group 7 and group 0.

SUNAYANA: As always, it might be handy to write some notes or diagrams along the way so hit pause when you need to. Don’t worry, we’ll still wait for you to hit play again.

TULELA: For this episode, it might be useful if you have a periodic table to hand in front of you.

SUNAYANA: Or if you have an amazing photographic memory. But before we delve into the properties of the elements in those groups, let’s have a quick reminder of why those atoms belong to those particular groups in the first place. Hi, NNICK. Please can you give us a summary of groups 1, 7 and 0?

NNICK: Groups are the vertical columns of the periodic table, running from 1-7 and finishing with 0. Don’t ask me why, it’s just a very human way of numbering things.

Group 1 are the alkali metals, so called because they are metals and because they react with water to form an alkaline solution. In chemical reactions, these alkali metals give away an electron.

Group 7 are the halogens, so called because they are halogens. In chemical reactions, these non-metals gain an electron to complete their electron shell.

And group 0 are the noble gases. Because their outer electron shell is already full, it is stable and the elements do not take part in chemical reactions. They are inert. How these gases can call themselves noble, I'll never know.

TULELA: Thanks NNICK! We’ll start with group 1 then. These are all the elements with one electron in the outer shell of the atom. Lithium, sodium, potassium, rubidium and caesium. These are known as the alkali metals and the further down the group the element sits in the table, the more reactive it is.

SUNAYANA: And this is because of that single electron in each of the element’s outer-shell. The further down group 1, the further away the outer shell’s electron is from the nucleus. The more electron shells there are shielding the positive charge of the nucleus from the outer electron.

TULELA: This means that the outer shell electron is less strongly attracted to the positively charged nucleus and so less energy is needed to remove it. This increasing reactivity becomes really clear when those alkali metals are put into water.

SUNAYANA: As we go down the group, the more reactive the metal is in water. Lithium, for example, will move around the surface, fizzing away until it disappears. Sodium and potassium do the same but progressively faster than lithium and also they melt away. In all three reactions, hydrogen is made and released and this can be tested for and will pop in the presence of a burning splint. You may have heard this called the "squeaky pop test". [POP]

TULELA: In fact, the potassium reaction with water is so vigorous that the hydrogen ignites spontaneously.

SUNAYANA: As well as hydrogen, the other product produced in each of these reactions is the hydroxide OH of these metals. For example, sodium hydroxide or potassium hydroxide.

TULELA: And when we test this with a universal indicator, the solution turns purple showing the presence of an alkali – which is why group 1 metals are called the alkali metals.

SUNAYANA: Makes perfect sense.

TULELA: That’s group 1, the alkali metals. Now, onto group 7. These are known as the halogens and include fluorine, chlorine, bromine and iodine – all with seven electrons in their outer shell. And because they are in the same group, they also have similar chemical properties.

SUNAYANA: They are all non-metals and exist as molecules of two atoms. So Cl2, Br2, I2 for example. As each element is one electron short of a full outer-shell, two atoms are needed to share one pair of electrons to get a full outer shell across the two-atom molecule. And there’ll be more of this sharing type bonding – covalent bonding – in series two.

TULELA: The further down group 7 the halogen element is, the higher its relative molecular mass, melting point and boiling point and the darker the element becomes. So, at room temperature, fluorine, a yellow gas, chlorine a green gas. Bromine is a red-brown liquid or orange vapour and iodine is a grey solid or purple vapour.

SUNAYANA: And as we go down the group of halogens, the reactivity of the element decreases. This is the opposite of what we saw with the alkali metals in group 1. And this is again due to how the electrons are arranged in the outer shell of the atom. The further down the group, the further away the outer-shell electrons are from the positively charged nucleus and so the harder is it for the atom to attract that extra negatively-charged electron to fill the outer shell.

TULELA: We can see this when halogens react with metals, for example the alkali metals with one electron in their outer shell. Those higher in the halogen group react more vigorously because they attract that outer shell electron from the metal atom more easily. And in each case the resulting compound produced is a salt – in this case called a metal halide.

SUNAYANA: So, for example, chlorine will react vigorously with sodium to produce the salt, or metal halide, sodium chloride.

TULELA: But if a more reactive halogen comes along, it can displace a less reactive halogen from solutions of its salts.

SUNAYANA: So for example, let’s say we have a solution of potassium bromide and we add chlorine solution. Chlorine is a more reactive halogen so will displace bromine to form potassium chloride.

TULELA: Yes – I like to imagine a pool party with groups friends in pairs chatting to each other – one a halogen and one an alkali metal. So it’s as if potassium and bromine are having a nice catch-up in one corner of the pool but along comes chlorine and displaces bromine who just swims off.

SUNAYANA: Shame – poor bromine. So halogens react with alkali metals to form metal halides and more reactive halogens displace less reactive ones

TULELA: And when the halogens react with hydrogen they form compounds called hydrogen halides which all dissolve in water to form acids. For example HCl or hydrogen chloride which dissolves in water to form hydrochloric acid.

SUNAYANA: Finally, there’s one last group we need to talk about – they’re the noble gases – helium, neon, argon, krypton and xenon in group 0.

TULELA: The noble gases have all got a full complete outer-shell of electrons and so are already quite stable on their own without requiring to gain or lose extra electrons. This also means that they exist as single atoms, unlike those halogens who need to pair up to form stable molecules.

As we travel down group 0 in the periodic table, the boiling point and density of the noble gases increase. And although they are inert, the noble gases have their uses. Helium is used in airships because it is less dense than air. Argon is used to protect metals being welded and the noble gases also give off light when an electric current is passed through them.

SUNAYANA: That’s group 1, group 7 and group 0 taken care of but if you’re doing triple science then you also need to know about the transition metals which sit in the middle of the periodic table between group 2 and group 3.

TULELA: They include lots of the everyday metals you may have heard of, including copper, iron, zinc, gold and silver. Although they have the same typical properties as all other metals – they conduct electricity and are shiny when cut – they tend to be more colourful and have other different properties to those we’ve seen in the alkali metals in group 1. You can find out more about these on the Bitesize website.

TULELA: How about a quiz about group 1,7 and 0?

SUNAYANA: Yes, please.

TULELA: As usual, three questions, five seconds each starting ….now. Question 1. In group 1 how does the reactivity of alkali metals change as we go down the group?

SUNAYANA: They get increasingly more reactive because less energy is needed to remove the outer shell electron from the alkali metal.

TULELA: Question 2. Chlorine is higher in group 7 than iodine. If I have a solution of potassium iodide and add chlorine to the solution, what happens?

SUNAYANA: The chlorine displaces the iodine to form potassium chloride.

TULELA: And question 3. Why are the noble gases so unreactive?

SUNAYANA: Because they have a full outer shell of electrons.

TULELA: Correct, correct and correct. Hope you did the same but don’t worry if you didn’t as there’s loads more hints, tips, diagrams and revision notes over at the BBC Bitesize website.

SUNAYANA: Quick summary blitz, Tulela?

TULELA: Lets go. Group 1 elements are the alkali metals, they have one electron in their outer shell and become more reactive the further down the group.

SUNAYANA: Group 7 elements are the halogens. They are one electron short of a full outer shell and become less reactive further down the group.

TULELA: Group 0 are the noble gases and they have full outer shells and so are inert or unreactive.

SUNAYANA: And for those of you doing triple science, remember those useful and colourful transition metals.

TULELA: Thanks for listening and remember you can listen on BBC Sounds to all the other episodes in this series, as well as many other Bitesize subjects.

SUNAYANA: In the next series, we’ll be looking at bonding in chemistry and how this affects the structure and properties of elements and compounds.

TULELA: Thanks for listening to this series.

SUNAYANA: Bye.