The combination of elements to give compounds. There are three laws of chemical combination.

1. (1) The law of constant composition states that the proportions of the elements in a compound are always the same, no matter how the compound is made. It is also called the law of constant proportions or definite proportions.

2. (2) The law of multiple proportions states that when two elements A and B combine to form more than one compound, then the masses of B that combine with a fixed mass of A are in simple ratio to one another. For example, carbon forms two oxides. In one, 12 grams of carbon is combined with 16 grams of oxygen (CO); in the other 12 g of carbon is combined with 32 grams of oxygen (CO₂). The oxygen masses combining with a fixed mass of carbon are in the ratio 16:32, i.e. 1:2.

3. (3) The law of equivalent proportions states that if two elements A and B each form a compound with a third element C, then a compound of A and B will contain A and B in the relative proportions in which they react with C. For example, sulphur and carbon both form compounds with hydrogen. In methane 12 g of carbon react with 4 g of hydrogen. In hydrogen sulphide, 32 g of sulphur react with 2 g of hydrogen (i.e. 64 g of S for 4 g of hydrogen). Sulphur and carbon form a compound in which the C:S ratio is 12:64 (i.e. CS₂). The law is sometimes called the law of reciprocal proportions.