The periodic table of elements

There are more than a hundred different types of atom - different elements. They can be grouped in different ways. Most of them are metals... A few are non-metals... Some of them react violently with water...

Some are gaseous and inert - not reactive at all. The properties of an element depend on its atomic size, but more importantly, how its electrons are arranged. This gives us a clever way of organising the elements. Let's start by placing the elements in a long row, from the atom with the fewest electrons to the one with the most. The electrons in an atom are grouped around its centre in electron shells.

Let's put those elements in a row of their own. The first two have only one electron shell. The atoms with two shells go in the second row... The three-shelled atoms in the third, and so on. Well, that's a start.

Now we have seven rows of elements: from the smallest - with one electron shell, to the largest - with seven shells. Look at the atoms we mentioned earlier, those that reacted easily with water. They ended up below each other in the same column. How about the gases that were non-reactive? Well, they are at the end of each row, but the rows are of different lengths, so they don't form a neat column.

Can we do anything about that? Let's look at the electronic arrangements of the atoms again. The thing that decides if - and how - an element reacts, is the number of electrons in the outermost electron shell of its atoms. In fact, the reason why these elements in the first column react in the same way, is because they have one thing in common. They all have one electron in their outer shell - one valence electron.

Now, if we slide some of the rows a bit, so that all the elements with three valence electrons end up underneath each other... We get some gaps in the middle. But look, by doing this, all the elements with four, five, six, and seven valence electrons also end up in columns. And so do the inert gases we talked about. That's because they all have eight valence electrons.

Oh, that's right. This atom here has only two valence electrons, but it is one of the inert gases, so it gets to slide over to that column as well. So now we have a table, where many of the elements that share properties appear in the same column. The rows tells you how many electron shells these atoms have. These columns tell you how many electrons the atoms have in their outer shell.

Now we have rearranged the elements, from a long line, into a table. When there was just one row of elements, certain properties appeared at regular intervals, or periodically. That is why this table of elements is called the periodic table.