All matter is made of atoms. There are more than 100 types of atoms, corresponding to the same number of elements. Examples of elements are iron, oxygen, calcium, chlorine, carbon, sodium and hydrogen. Most matter consists not of pure elements but of compounds: two or more atoms of various elements bonded together, as in calcium carbonate, sodium chloride, carbon monoxide. The binding of atoms into compounds is mediated by electrons, which are tiny particles orbiting (a metaphor to help us understand their real behaviour, which is much stranger) the central nucleus of each atom.
