In this article, we hope to develop an understanding of atomic structure and the periodic table, from the beginning of secondary school/junior-high level, to first/second year undergraduate physics/chemistry, via one fictional conversation.
We hope that people at any stage of that journey will find a section that speaks to them. We would love it if it turned out to be useful to teachers, too. It is not a history of discoveries about the atom; it is a conceptual, rather than historical, journey. It might be worth having a periodic table available while reading…!
SIGRID: I’ve been reading that everything is made of atoms, and that the types of atoms are shown in a diagram called the periodic table. It all seems a bit weird, so I want to know more. I’ve got a load of books, and I’ve ordered them by complexity, but I’m not sure I can just teach myself this stuff. Can you help?
SALLY: OK, I’ll try. What do you know so far? For example, draw me an atom.
SIGRID: What do you mean, draw you one? It’s like a ball. I’d just draw a circle.
SALLY: And what’s the Periodic Table?
SIGRID: A list of different substances. But there are only about 100 in it. And I know there are loads more than 100 substances. Sugar, salt, wood, nitroglycerine… They’re not in there.
SALLY: That’s because the periodic table (and here’s one from the Royal Society of Chemistry) only shows elements. Other substances are compounds, or mixtures of elements and/or compounds. A compound is two or more elements chemically combined in fixed proportions.
SIGRID: OK so you’ve explained compounds in terms of elements, but I don’t know what an element is.
SALLY: In an element there’s just the one kind of atom.
SIGRID: So there are different kinds of atom, one for each element?
SALLY: Yes, and compounds have more than one kind of atom, chemically combined, often into groups of atoms called molecules.
SIGRID: So are you saying that compounds are made of molecules? Because we talk about oxygen molecules and nitrogen molecules. And they’re elements – look, I can see them in the periodic table. Element numbers 7 and 8.
SALLY: Good point! Molecules can form elements or compounds, just depending on whether the atoms are the same type or not. In fact I often use the word ‘particle’ to cover both atoms and molecules to get over that problem, unless the difference is important to what I am describing. This is potentially confusing, so why don’t you find out about and draw the structures of helium, nitrogen, oxygen, carbon dioxide and air, to explain all these terms.
SIGRID: OK, here goes.
Here’s my picture. The first three are in the periodic table because they only have one kind of atom each (and the fact the second and third are grouped into molecules doesn’t matter). The fourth one is a compound and the fifth is a mixture, so they are not in the Periodic Table.
SALLY: Exactly. And that’s why you won’t find sugar, salt and the others in your list in the periodic table. The periodic table is just reserved for elements.
SIGRID: One kind of atom…
SALLY: Yep.
SIGRID: In the pictures of molecules, the atoms are joined together. How does that work?
SALLY: Ah, that’s called chemical bonding, and we’ll have another conversation to cover that. But we’ll need to have this discussion about atomic structure and the periodic table first. For now it’s enough to know that chemical reactions involve a rearrangement of atoms, but there are still the same number of atoms, and of the same elements, before and after a chemical reaction.
SIGRID: So if we burn carbon completely in oxygen to make carbon dioxide, a picture might look like this, and there are the same number of dots of each colour before and after.
SALLY: Precisely. That’s pretty much how Dalton got his ideas of atoms in the first place. In the reaction you mentioned, 3 g of carbon will always react with 8 g of oxygen to make 11 g of CO2. Those fixed ratios make perfect sense when you consider that it is all to do with rearranging dots like in your picture.
SIGRID: OK, that all makes sense, but I don’t think I can learn about 100 different types of atom.
SALLY: You don’t have to.
SIGRID: How come?
SALLY: Can you spell 100 words?
SIGRID: Yeah, of course. Thousands maybe. Don’t be silly.
SALLY: And how is that possible?
SIGRID: Well, the words are made of letters and there are only 26 of those.
SALLY: And atoms are made of other things, like words are made of letters. But atoms are much easier. There are only 3 things they are made of instead of 26. Also, they also follow patterns and rules, which will help you work things out when you can’t remember them.
SIGRID: What do you mean, ‘they are made of other things’. In this book it says that an atom is the smallest part of an element that can exist.
SALLY: Yes – just as molecules are made of atoms, so too atoms are made of other things. Let’s cut to the chase and call them protons, neutrons and electrons. What your book is saying is that atoms of, say, lithium, neon, gold and uranium are all different, but a proton in a lithium atom is identical to a proton in a neon atom; an electron in a gold atom is identical to an electron in a uranium atom.
SIGRID: So you don’t have a lithium proton and a neon proton.
SALLY: Exactly – protons are protons. What makes a lithium atom different from a neon atom is the number of protons, neutrons and electrons in the atom. Specifically the protons actually. What makes lithium lithium is that its atoms have three protons.
SIGRID: Every single one?
SALLY: Every one.
SIGRID: But there must be trillions of atoms of lithium in the universe. They can’t all have three protons.
SALLY: They do! Because an atom that doesn’t isn’t lithium! That’s what makes it lithium. We could call it ‘the-element-with-three-protons-in-its-atoms’ but that’s not as catchy.
SIGRID: OK, we’re arguing about protons, neutrons and electrons, and I don’t even know what they are.
SALLY: They are particles that make up atoms.
SIGRID: Hang on, you said that atoms and molecules were both particles. I let that go, but I didn’t like it much, because we had particles being made of particles. Now we have particles being made of particles that are made of particles. Why don’t you just invent a word that means what it says?!?
SALLY: Fair point. I guess we use the word ‘particle’ as it suits us. It sort of just means ‘a useful bit of matter on whatever scale we are talking about’. So if we are looking at gas pressures, we might talk about atoms and/or molecules as particles, because they are the things whizzing round and doing the stuff that we are interested in.
SIGRID: Whereas for our new understanding of the atom, we are looking one layer deeper, so the protons, neutrons and electrons are the ‘particles’ of interest.
SALLY: Exactly. How it was discovered they even exist, let alone make up atoms, is a huge story for another conversation. You’ll just have to accept their existence for now.
SIGRID: Hmm. I suppose.
SALLY: Now, to apply this abstract idea, it would be helpful to have a picture of an atom that’s one layer deeper, as you describe it, than the picture of an atom as an indivisible sphere.
SIGRID: Yeah, there’s one in this book. The protons and neutrons form a central part, called the nucleus, and the electrons seem to orbit it, like the planets orbit the Sun.
SALLY: Right, and if you asked a member of the public to draw an atom, that’s probably what they would give you. We are going to get more sophisticated eventually, but this is a really useful stopping off point. Just bear in mind that this diagram can’t be drawn accurately – to make it to scale, with the nucleus that size, you would need a piece of paper the size of a swimming pool to fit the electrons on.
SIGRID: But that means an atom is mainly empty space!
SALLY: Crazy, eh? And since you are made of atoms, you also are mainly empty space!
SIGRID: Now, look – there’s also a table in all of the books I have found. It gives the masses and charges of the protons, neutrons and electrons.
Does the last picture and the table mean that protons are yellow?
SALLY: NO! A proton is not yellow! It’s not any colour. They are too small to even have a colour because they are way smaller than the wavelength of light. But people like to draw pretty pictures, and in the future some idiots are going to put this conversation on their website, so we are just matching their colour scheme…
SIGRID: And do we worry about the accurate masses?
SALLY: Probably not for this conversation –the proton and neutron masses are so similar that we’ll imagine they are the same, and the electron mass is a lot, lot less than either of those. But it would be useful to bear in mind that the electron mass is not exactly zero (if it was, electrons would have to travel at the speed of light), and when you study radioactivity or particle physics, you will need to know that the neutron has a mass slightly greater than the proton.
SIGRID: I have to say, the table still doesn’t really help me feel like I’ve got to know these things very well.
SALLY: Really? Let’s try something else first. How would you describe how people are different?
SIGRID: Well, there are loads of ways. They have different likes and dislikes. They look different. Some are taller, some shorter. They have different skin colours. Different genders. Some are calm, some irritable. Some kind, some mean.
SALLY: Exactly. People are complicated. Particles aren’t. There is a very limited number of ways that protons, neutrons and electrons could in principle be different from one another. For now, mass and charge are going to be the two we concentrate on. But there really aren’t all that many more…
SIGRID: And what does differing mass and charge do to how they behave? I mean, I know that the charge of an object tells you how much force it will experience in an electric field, and the mass tells you how much force it will experience in a gravitational field.
SALLY: That’s a good comparison! But these things are too tiny for gravity to be important. So what else does the mass tell you?
SIGRID: Oh, how much it is accelerated by a force. Like the attraction and repulsion of charges, I guess.
SALLY: Yes! And these things are overwhelmingly in the grip of exactly those electrical forces.
SIGRID: The neutrons aren’t. They can’t be. They have no charge.
SALLY: True. They help to glue the nucleus together using something called the strong force, otherwise it would fly apart due to the electrostatic repulsion of the protons.
SIGRID: Because positives repel other positives.
SALLY: Yes, but the strong force is very short range, so the nucleus as a whole interacts with electrons via electrical forces. So going back to the mass/charge thing, what can you tell me now?
SIGRID: The electrons and nucleus will be attracting each other because one is positive and the other negative. And Newton’s Third Law tells me that they will attract each other with equal forces. But the effect of those equal forces will be very different. An electron has a tiny mass, so it is affected much more by that force. It orbits the comparatively heavy nucleus, rather than the other way round. Just like the planets orbit the Sun, not the other way round.
SALLY: Exactly.
SIGRID: OK. So, it’s the protons and neutrons that give the atom almost all its mass. And the protons that tell you which element it is. How do I know how many electrons there are?
SALLY: An atom is electrically neutral.
SIGRID: Oh. Protons and electrons have equal and opposite charge, so in an atom the number of electrons and protons must be equal so that the charges cancel out. And what’s the point of electrons?
To be continued… Find Part 2 here.
And in the meantime, there is eternal honour and glory (and absolutely no material reward whatsoever) for anyone who can tell us why the characters are so named…