The atoms of an element are all exactly alike. They weigh the same and act the same whatever their source. Two or more atoms of an element may combine to make a molecule of that element. The molecules of a chemical substance are always composed of the same number and kind of atoms.

To express the composition of substances chemists have made use of certain abbreviations and signs. To indicate an atom of hydrogen the letter H is used and for oxygen, the letter O, for nitrogen, N, and so on as shown in the table.

When expressing a compound the number of atoms is indicated by sub-script; for example, H2 means two atoms of hydrogen; H20 expresses two atoms of hydrogen and one atom of oxygen, and as we have found, this is the composition of water; so H20 is the chemist's short way of indicating water. These are called chemical formulas. The formula for sulphuric acid is H2S04. This indicates that it is made up of two atoms of hydrogen, one atom of sulphur, and four atoms of oxygen. The following table gives the chemical formulas of many of the chemical substances found in the household.