Periods and groups

The periodic table is essential for all chemists and an important tool for predicting how the elements will react. But how were the elements we know and love actually discovered in the first place? And who put the table together? Chris Smith spoke with chemist Peter Wothers from the University of Cambridge, asking firstly, where did it come from?

Peter - Well actually it took quite a while to develop. I mean it didn't just appear in one go, and actually a number of people independently discovered this all around the same time actually in the eighteen sixties, but there was a long time leading up to that different forms began to emerge in a sense.

Chris - How did people realise that there was this relationship between the elements that we see around us and that we could arrange them into a table like that?

Peter - Well people were beginning to look for some sort of structure in the elements and the similarities that they had way before the periodic table itself came about. So for instance there were certain elements that were recognised as having very similar properties. Some of these for instance were these so-called ‘alkaline Earths’ and actually even before they were identified as elements in our modern sense, their compounds were recognised as having very similar properties. And eventually when the metals were discovered they were called the ‘alkaline Earth metals’ because again they all shared similar chemical properties. Similarly the ‘alkali metals’ as you heard they all fizz and react with water very violently. So that whole group were recognised as something that were very similar to each other. And so then people began to look for ways that these similar elements might be put together in an arrangement.

Chris - If one looks at the periodic table, so far over to the left, that's what we call group 1 and they are the elements, the alkali metals you've been mentioning, that fizz when we drop them in water. What's the difference between the ones right at the top left and as you go down that block on the far left hand side. How do they change physically the element?

Peter - Well they change in a number of ways I mean that the most basic thing is actually that they get heavier as you go down the group. So the individual atoms are increasing mass. In fact lithium, sodium, potassium are all light enough that they would float on water but actually rubidium and caesium would sink in water, but then they would react very violently in it. But there's also more subtle properties as well. So for instance how easy it is to remove that outermost electron that all of those elements share becomes easier and easier as you go down the periodic table. That means in a sense that those metals become more reactive.

Chris - So the ones on the far left are all metals. As we move across to the right you get into the group next to group one the alkali metals and they're a bit less reactive aren't they. Things like magnesium but they behave in a sort of similar way and then as you go across what changes?

Peter - So as you're moving across the periodic table what is happening is you're adding one extra proton in the heart of the nucleus. And so this in a sense will hold all of the electrons sort of more tightly in, and so then it's harder to remove the two electrons from the next group and actually by the time you've got all the way across the right hand side it's very very difficult to remove any of those electrons. And is actually their properties the more that they would accept electrons they would gain electrons and become negatively charged ions in some cases.

Chris - I remember my chemistry teacher putting a sort of ruler across the periodic table on the far right hand side and saying everything to the left of this is a metal and the things to the right of this ruler are non-metals and there are far more metallic things that behaves as metals, than there are non metals. Why is that?

Peter - That's quite difficult actually it's all to do with the number of electrons that each atom has and how they can form bonds. And actually you only get molecules within the elements for the right hand side of the periodic table, when for instance you get two oxygen atoms forming an oxygen molecule. But for the other ones you get larger structures with fewer electrons trying to form weaker bonds between all those. That's a typically a metal really with sort of these more what we call them delocalised electrons, electrons that can move around more easily throughout the whole structure.

Chris - Now when Mendeleev put his table together it didn't look like the one we have today. He actually left gaps. Was that because he realised there was this this sort of periodic behaviour? Things did follow a pattern and a sequence of behaviour and so he knew that even though we had discovered something yet there had to be something that would fit with that sort of position in the table. Is that where those gaps originated?

Peter - Well of course I mean Mendeleev wasn't the first to draw up his periodic table and others before him also left gaps for elements that were yet to be discovered. I mean the real difference with Mendeleev and his periodic table is actually that he very accurately predicted the properties of those missing elements and their compounds. And when they became discovered that's what really drew people's attention to his system.

Chris - When was the sort of golden era for discovering elements? When was the gold rush that loads and loads of them began to appear and fill in the table?

Peter - Well to fill in the table you had to have a table but they do come in sort of spurts. But I mean one of the key things actually in the late 1850s and early 1860s was the discovery of spectroscopy. So that revolutionised the discovery, because remaining elements at that point were very rare in a sense, so hard to find. And so you needed a very, very sensitive technique and that was the sort of flame test idea that allowed some elements to be discovered all in one go.

Chris - I'm glad you brought that up because we're going to hear about the pioneer of that, Robert Bunsen next. But sitting in front of you you have something exciting that you wouldn't reveal to me earlier. What's that?

Peter - So this is actually a copy of the very first periodic table which is seven years before Mendeleev's version. You say that Mendeleev's version looks very different from the modern version. Well this one looks completely different. This one was designed to be wrapped around a cylinder and actually it has a continuum of the elements arranged by their weight as Mendeleev also arranged them of course. And on this version the elements with similar properties also align vertically in groups.

Chris -Where did you get that?

Peter - So this one is one that is owned by St Catherine's College and will be on display there in March and April this year.

Chris - In line with the 150th anniversary of the periodic table as a concept?

Peter - That's right. We will be showing this very first form of the periodic table. And indeed a number of other versions including Mendeleev's first versions as well.