Episode 3 – Atoms and the Periodic Table
Welcome to Untangling Science, a podcast about science that is for everyone, with me Darragh Ennis. You probably know me as The Menace from ITVs quiz show The Chase, but my day job is as a scientist at the University of Oxford. In this podcast I want to bring the world of science to people who think it’s too complicated to understand in a way that is fun and straightforward. We have a website, www.untanglingscience.com and you can follow us on Twitter @untanglings . I have a blog on the website that I leave useful information, links and diagrams from each episode in, so check that out. In this episode I will be leaving my own comfortable world of biology to dip our toes into chemistry. Please note, if in a chemistry lab, don’t actually dip toes or other body parts into anything. Today we will talk about Atoms and the periodic table. I think these are fairly fundamental topics but we will phrase them as questions. First up is: What actually is chemistry? What are atoms and molecules? What are elements and how did they end up in that weird looking table that was on the wall in my science class?
If any of you follow me on Twitter, you will know I love a bad joke so I will get it out of the way before we get working on answering those questions. You should never believe an atom, they make up everything…. I know that’s a joke, but really it’s true. Atoms are the tiny parts that pretty much every tangible thing is made of. And if we ask the first question, “What is Chemistry?”, this is a good place to start. I’m fairly sure that a large proportion of people listening to this are already picturing chemistry like we see in movies or on the covers of college prospectuses. There’s likely one or two people in white coats, holding up a glass container and looking at some blue liquid. You might be thinking about fantastic glassware all linked up with rushing liquids and steam flowing through them or maybe even gigantic explosions. To an extent some of all that happens but the definition of chemistry we are going to use is a lot simpler and it was the one I was given on my first day of university. Chemistry is the study of matter. That’s a very neat and simple explanation that is also quite true, chemistry is the study of well stuff. If it exists and takes up physical space and has mass then that’s chemistry. So if you ever hear anyone say something like “I don’t eat that, it’s full of chemicals”…. Chemistry being the study of stuff is fine, but the problem is that stuff is incredibly variable. If we’re to study everything from water, umbrellas, kangaroo fur and from moss and stars to my mam’s Rolling Stones records we might need to find something in common across all of stuff. We do that by going fairly deep into what they’re made of. At some level all of stuff is made from the same, well stuff. And those we call atoms.
The atom was first considered as a concept, like an awful lot of other things, in Ancient Greece. Seriously, those beardy philosophers were really amazing with thinking about how the world works. Well about 2,500 years ago one of them, Democritus, got to thinking about how long you could go on cutting something up. He reasoned that there had to be a point where you couldn’t divide a thing any more, where it would be at its smallest and most basic point of existence. He named this the atomos which literally meant something that can’t be split. We figured out that he wasn’t quite right about atoms not being splitable, more about that later, but in the idea that there was a point when a substance was at its very basic form he was quite right. Atoms can indeed be broken down into smaller parts but once you go below the atomic level all of the characteristics of that atom will disappear. So an atom of say gold or oxygen still acts like a tiny piece of gold or oxygen, but the smaller parts that make them up are identical across all atoms, only the number of them changes. But what are these parts? How were they discovered?
The first of our particles to be discovered is the smallest one, the electron. At the end of the 19th century scientists were working on cathode rays, a strange phenomenon you could observe when running an electric charge through an almost complete vacuum. The vacuum tubes were created by filling a glass tube with a gas and then removing almost all of the gas, leaving only a trace amount inside the tube. If there were electrodes at either end of the tube then a voltage applied would cause a glow to appear, though there was no satisfactory explanation of the cause of this. In 1897, a British physicist called J.J. Thomson suggested that the rays were actually made of a particle even smaller than the smallest atoms. As they were moving from the negative electrode to the positive he showed that these particles were also negatively charged. Thomson called them corpuscles but the term electron was adopted very soon afterwards by the physics community. He suggested that they were part of the atoms of the small amount of gas remaining in the tubes and showed that he got similar results from many different gases. This meant that electrons seemed to be part of the atomic structure of all the elements. Thomson was also able to calculate that electrons were about 1000 times smaller than the smallest atoms and could be deflected by careful application of electrical or magnetic fields. This last point became critical in the use of cathode ray tubes as this was the basis of early television broadcasting, as this allowed cathode ray tubes to project an image onto the screen of early TV sets. Indeed, until the invention of transistors in the 1950s cathode ray tubes were at the heart of radio, TV and the whole of electronics as a modern scientific field. Anyway, back to J.J. Thomson.
Thomson then proposed an idea for how atoms are made up which is now universally called the plum pudding model. Plum pudding is not as popular now as it was in the late Victorian era, but if you think of a fruit cake with lots of raisins like a Christmas cake that’s probably close enough. Though no icing or marzipan or little sugar reindeer on top, pretty sure that’s not on atoms. Anyway, if we think of a plain fruit cake Thomson suggested that because electrons are negatively charged, which he knew by how they behaved in the cathode ray tube experiment, but atoms themselves are neutral, this suggested that some other part of the atom must be positively charged to balance it out. His idea was that there was a positively charged mass of the atom with the negatively charged electrons embedded in it like raisins are embedded in a fruit cake.
The plum pudding model was later found to be incorrect and a new model of the atom was developed by a brilliant New Zealand scientist working in the UK, Ernest Rutherford. Rutherford came up with this model after he conducted one of the most famous experiments in physics the gold foil experiment. This work he did with two other famous scientists of the time, Geiger and Marsden. And yes, it’s the same Geiger as in the Geiger counter that you think of from movies that clicks when there’s radiation around. This experiment worked by firing positively charged particles at an extremely thin gold sheet. Gold can be made formed into sheets that are extremely thin, which was pretty vital for this experiment, as they wanted to see what atoms did and the sheet was only a few atoms thick. Now if we think of a magnet, we know that if you try to put the two positive ends together they will push apart. The same is true with tiny particles and any two positives will bounce away from each other. In the plum pudding model the electrons balance out the positive charge of the rest of the atom that they are embedded in. If this were true then the positively charged particles that Rutherford’s team fired at the super thin gold foil should pass pretty much straight through. And for most of them, this was the case. A small number of the particles were detected as being deflected off to one side or even back the way they came. So what did this mean? Rutherford formed a new model where the atom was formed of a small positively charged nucleus at the centre surrounded by a sort of shell with electrons and quite a lot of space in between. In later years when conducting similar experiments on nitrogen gas, Rutherford discovered that positively charged particles could be extracted from nitrogen and other elements and he named these protons.
The last of the components of an atom that we will talk about today, the neutron was discovered some years after the proton by an English physicist called James Chadwick. The experiments involved some quite complex work with radiation so we’ll make it as simple as we can. The study of radiation was still in its early days at this point, and one form of radiation was able to eject protons from atoms. The radiation was not electrically charged so it was clear it wasn’t formed of negatively charged electrons. Chadwick was also able to show that the particles in this radiation weighed about the same as a proton, so far heavier than electrons. He named them neutrons and correctly suggested that they formed the nucleus of all atoms. Neutrons would soon become of great interest as it is through neutrons that we can add a very significant two letters to atom and it becomes atomic, but more on that later.
Okay, we’ve covered quite a bit and there was actual physics in there so let’s sum up what we need to know to move forward.
- Chemistry is the study of stuff, and all stuff is made up of atoms
- An atom is the smallest piece of something that still has the same properties
- Atoms are made up of 3 types of particle, electrons, protons and neutrons
- Electrons are super tiny and are negatively charged
- Protons and neutrons form the atom nucleus.
- Protons are positively charged, neutrons have no charge.
So now we know that every atom is made up of a nucleus (scientists love the word nucleus, it crops up a lot in different fields but it just means the centre) with some number of protons and neutrons in it. Then outside this nucleus are some electrons, and the vast majority of most atoms are just empty space. About 99.999999999999% of atoms is empty space. To give you an idea of what this means, if we expanded a hydrogen atom up to the size of the earth, the nucleus at the centre would be about the size of the millennium dome in London. Floating around in all of this empty space are one or more electrons. Now, you can go even smaller to the quarks that make up protons and neutrons, but we’ll forget about those for now and think only in terms of protons, neutrons and electrons. So if all atoms are made of the same things, how do we have different chemical substances, why isn’t everything all the same? Well it turns out that the exact number of those three components of atoms is critical in deciding what the atom is like. These configurations of protons, neutrons and electrons are able to give atoms a unique set of characteristics that we call elements. And in fact, it’s useful to think of an atom as the smallest amount of an element that still has these characteristics, and an element is simply a substance that only contains one type of atom. In ancient times there were believed to be four elements, earth, air, fire and water and in the 1990s they believed if you added a fifth element, heart, you would form an environmentally conscious superhero called Captain Planet. These days though there are, at the time of recording, 118 chemical elements. I say at the time of recording because new ones get discovered every few years and they are officially named, given a one or two letter symbol and placed where they belong on something most people recognise, the periodic table.
Now the periodic table might give some of you nightmare style flashbacks of school and exams you didn’t really study for but it’s nothing to be afraid of. While we’re talking about it it might help to have one somewhere you can look at, but obviously not if you’re driving or something. The periodic table might look weird and complicated, but it really is a very clever way to show the elements and give you information about what those elements are like and their properties. And the really clever bit about it is that the guy who invented it left gaps for those elements yet to be discovered as he knew something would be around that fit the table he had devised. For me this is one of the most far sighted scientific discoveries of all time and it’s inventor Dmitri Mendeleev has element 101 named in his honour. Mendeleev was born in 1834 in Siberia, the youngest of 17 children. He was a professor of chemistry at St. Petersburg University by the time he was thirty and established a world renowned chemistry research group. While studying for a text book he was writing, he began to notice patterns in the chemical properties of the 56 known elements at the time. One morning he wrote out a table to group the elements based on these patterns that were regular or periodic, hence the name. He later said it came to him in a dream and is quoted as saying: I saw in a dream a table where all elements fell into place as required”. While other researchers had proposed periodic tables, they all had substantial flaws and in any case Mendeleev would not have been aware of this work as it was primarily by western scientists and not yet translated to Russian. Mendeleev ordered the elements according to their atomic weight and when this was done there became clear patterns in their properties. The atomic weights were mostly evenly spaced and where there was some larger space he predicted, quite correctly, that new elements would be found to fill those gaps. When Gallium and germanium were discovered a few years later they fit exactly into Mendeleev’s table. It really was truly remarkable.
Now what of the table itself? What do all the numbers, rows and columns mean? Let’s start with the entry for each element. All 118 elements so far are represented by one or two letters. These are universally acknowledged and are the standard used in chemistry across the world. Above the letters is the atomic number which is equal to the number of protons in the nucleus of that element and it’s by this number that we can order the elements on the table. The number of protons is also what makes something be like that element. Changing the number of neutrons or electrons has other effects, but every atom with say 6 protons is always a carbon atom. On some periodic tables there is another number under the chemical symbol that represents the atomic mass. This is usually the number of protons and the number of neutrons plus or minus a little bit. So Hydrogen, which has 1 proton and no neutrons has an atomic number of 1 and an atomic mass of 1.00784. Helium is element number 2 and has 2 protons and 2 neutrons and an atomic mass of 4.002. So why is there a little bit? Well while the number of protons is crucial to determine what element an atom belongs to, the number of neutrons isn’t. For various reasons that we won’t get into, most elements have some atoms in nature that have a different number of neutrons in their nucleus. So the atomic mass is the average mass of atoms of that element in nature. When an element has the same number of protons but different numbers of neutrons this is called an isotope. I know that’s a technical term but I just want to add it in in case you wondered where the name of the baseball team in the Simpsons comes from.
Ok, so now we know what the letters and numbers for each element on the table mean, what is it such a weird shape? This is to do with the periodic way that the elements are similar. The table is arranged in 8 groups that are columns, so they are ones that are on top of each other. All atoms have what are called shells of electrons surrounding the nucleus. It’s helpful to think of these as being rings around the nucleus in the same way that planets are in rings further and further from the sun. This is absolutely an over simplification of the real situation, but we’ll stick with it for the moment. Each of these shells can have multiple electrons though, with the inner shell having two and the further out ones having eight. The groups are based on how many electrons are in the outermost of the shells in it’s atoms. So in the first column, Hydrogen at the top has one electron only, so it has one in the outer shell. The next one down in that group is element 3, lithium which has 3 electrons. The first shell is full with two so the outer shell also has one electron. And so on down the group, every one having only one electron in the outer shell. On the opposite end of the table in what are called the noble gases the outer shell of all of them is full. Ok, so why does this matter, you said it was all about protons! Well it is, but protons determine what element an atom is, NOT how that atom will behave. And when it comes to how it interacts with other atoms it’s electrons that are the key. But before we get into that, it’s time for a recap.
- Anything that has only one type of atom is an element
- Elements have certain characteristics
- Some elements have very similar characteristics and if you sort them by atomic number this similarity forms a pattern
- Mendeleev figured this out and devised the periodic table to show it
- Atoms have their electrons arranged in shells, and the number in the outer shell is important for some reason
Back to electrons and things. Why does the number of electrons in the outermost shell have any impact on what an element is like? To answer that I want you to consider how common it is to find atoms of elements in their pure form in nature. Almost none of the 118 discovered so far do that. They are almost all found mixed in with some other element. This is because of what we will call stability. Atoms like having their outermost shell full. Ok, they don’t like anything as they’re not alive, but it’s a state that most end up in. When the outer shell is full, those atoms are very unlikely to react with any other atom. In order to do this the vast majority of them will either share, take or give one or more electrons to another atom so they are no longer just an atom they are now a molecule. And a molecule is just two or more atoms linked together by sharing electrons. Most molecules in nature are stable as their electron outer shells are full because atoms will keep reacting with other atoms until they become stable and can’t react any more. Atoms do this in one of two ways, they either share electrons or they give them away. I’ll give some examples of both to tell you what I mean. The first one we’ll talk about is Oxygen. Oxygen is almost never found in nature in a single atom as it has two empty spaces in its outer shell so it really wants to fill those. Most of the time Oxygen gas forms a molecule with another oxygen molecule to form O2. And by this I mean the gas and not the mobile phone company. How this happens is that the oxygen atoms partner up with another one and shares two of it’s electrons. So each one has four of it’s own outer electrons and shares 4 with the other one so they both have 8, the key number. The fact that they share these electrons bonds those atoms tightly together to form a stable molecule, and as they now don’t have any gaps in their outer shell they are very unlikely to react with another atom. These kind of bonds are much more likely to form when both atoms need a small number of electrons to fill the outer shell and become stable. The other type of bond, where the atoms give away or take electrons happens when an atom that needs to lose a small number of electrons gives them to an atom that needs a small number. Again I’ll use an example you’ve heard of and that is common table salt, NaCl. Sodium has one electron in it’s outer shell and needs to lose that to become stable. Chlorine is one short of a full outer shell and needs to gain one, so these two fill all the criteria of the chemistry dating app algorithm. Sodium donates its electron to the chlorine and this has an effect on their electrical charge. If you remember, electrons are negatively charged, so sodium losing an electron makes it positive and chlorine gaining one makes it negative. And as Paula Abdul famously sang (younger listeners may need to look this one up), opposites attract. The two atoms are electrically attracted and form a strong bond to make salt.
So on the periodic table the columns are all linked by how they react with other elements and this occurs in a repeating pattern due to the number of electrons available in the outermost shells of those atoms. But what happens if the outermost shells are already full? Well those are in the column on the right hand side of the table and are called the noble gases. Noble gases are called this because they are a class apart and tend not to mix with the rest. They used to be called the inert gases but some of them can react under very specific circumstances. In general though those elements are pretty unreactive and many can be found quite often as non-bonded atoms in nature. But it’s not just reactivity that can be found in patterns on the periodic table. As you go across the table from left to right they are grouped in the type of thing that they are, with different metal types on the left and centre, to metalloids that are like metals in many ways but have different properties like silicon. Then there are the non-metals like carbon and oxygen before finally we reach the noble gases. The rows of the table, so those lines going left to right, all have the same outermost shell for their electrons. So the first row only have one electron shell, the second has two etc.. Finally, the higher the atomic number on the table the more likely we are to run into radioactive elements. There are a few earlier ones, but pretty much everything from element 84 to 103 are radioactive. So an element’s position in the table and the position of those around it can tell you quite a lot about how the atoms of the element behave.
We’re nearly at the end so I’d like to talk to you about some research and this episode it’s about how new elements are discovered, but first a quick recap
- Atoms are stable when their outer shell of electrons is full
- When they’re not full they share electrons with other atoms, or take/give them so the outer shell gets full
- This is called bonding and its how molecules and compounds are formed
- On the periodic table the columns show how many electrons are in the outer shell, and the rows show how many shells the atom has
- The different areas of the table are grouped into similar elements like metals, non-metals and the noble gases.
Ok, last of all we dip into some research and this time we will talk about how new elements are discovered. In the old days it was a case of finding elements in things that are lying around, and in fact four elements were discovered in and around one small town in Sweden called Ytterby. The tricky part was figuring out what was an element and what was just compounds of ones already discovered. These days it is very different, all of the elements that are found in nature have been discovered and the early parts of the table have been filled in. Now the goal is to add more to the end of the table and that offers a very different challenge. Once you get to this end of the table, even having a full outer shell of electrons isn’t enough to make an atom stable, they all degrade very quickly into elements further down the table. So once we get past about element 100 or so they are all made in labs and often only exist for a very small period of time. How it’s done is extraordinarily complex but I’ll give you a simple version. Particle accelerators are machines that use magnets and other forces to accelerate particles like protons and neutrons to extremely high speeds. The most famous particle accelerator is the large Hadron collider at CERN. They are ring shaped and fire beams of particles at immense speeds and then collide them. This can, under the right conditions cause new protons to be added to atoms nuclei and then form a new element. The scientists then have the tricky problem of proving that a new element has formed as these new heavy elements don’t last long and are made in really tiny amounts. Once it’s accepted that it’s been made the International Union of Pure and Applied Chemistry have to approve a new name and then it’s placed on the periodic table.
So what have we learned today? Time for a final recap
- Chemistry is the study of stuff and stuff is made of atoms
- Atoms are themselves made of protons, neutrons and electrons
- Anything that only has one type of atom is an element
- The periodic table arranges all the elements based on their chemical properties
- Atoms interact to have full outer shells of electrons, which makes them more stable
- This is how chemical bonds form to make compounds and molecules
- New elements are made by smashing particles together in particle accelerators
So that’s all for this week. I hope you enjoyed it. Please make sure you subscribe and help to spread the word. I have planned to do ten episodes of this podcast to see how it does and if you want more after that I will need to get the listener numbers up so let people know, leave reviews and get your friends to listen. Next week we will leave the cosy confines of planet earth to talk about the true star of the show, the Sun. We will talk about how it was formed, what is actually burning up there, how it compares to other stars and how scientists believe it will end. There is a transcript on the blog post for this episode on www.untanglingscience.com and comments are always welcome. As always thanks to Neal from PodKnows Productions for editing and all his help and advice and to Paul Farrer for his super cool original theme music. Talk to you soon