 But what are those particles? Well we call them atoms. Atoms are ridiculously tiny, about one times ten to the minus ten metres across. That means that you can fit about six million of them across the width of a human hair. Atoms are themselves composed of even tinier particles, which we call subatomic particles. Those are protons, neutrons and electrons, and they're arranged roughly like this. The protons and neutrons are clumped together in the nucleus in the centre of the atom, and the electrons are whizzing around the outside. This is a simplified picture, and we're going to get into more detail about what an atom is really like later on, but it'll serve us just for now. For the moment though, let's just represent single atoms as balls, since they are roughly spherical. Different atoms have different sizes and masses, and this has to do with the fact that they're made of different numbers of protons, neutrons and electrons, so some are lighter and some are heavier, some are bigger and some are smaller. As I said, we'll go into that in detail later on. If you start joining atoms together, chemists say the atoms are bonded together. If you join them together into groups, then those groups are called molecules. So a molecule is any group of two or more atoms that are bonded together. Some substances are made up of individual atoms, while others are made up of molecules.