Chemical elements

A chemical element is one of those squares on the periodic table that adorns many a classroom wall. Sure. But what is an element really? It is a more subtle question that you might realise.

To get to grips with it, it helps to first understand that atoms, the basic units of all matter, are made of protons, neutrons and electrons. It is the number of protons, otherwise known as the atomic number, that determines which element any given atom represents. If an atom has 2 protons, that’s an atom of the element helium. If it has 7 protons, that’s nitrogen.

But the logic of the periodic table also relies upon the number and organisation of electrons in an atom. Let’s take chlorine as an example. It is positioned in group 7 of the periodic table because it has seven electrons in the outermost shell of electrons. The trouble is that chlorine almost never exists in this form and so the atom depicted on the periodic table is almost an abstraction. In its simplest form, chlorine often exists as a gas composed of pairs of chlorine atoms joined together sharing their electrons.

We also, confusingly, tend to refer to pure substances made of one kind of atom, like chlorine gas, as elements. But these are very different from the isolated atoms depicted on the periodic table. That’s why some chemists think we should make a distinction between the abstract chlorine on the periodic table and chlorine gas, which we could call a ‘pure substance’ instead.