A compound is a substance that contains two or more elements chemically combined in fixed proportions by mass. The weight of a compound is the sum of the weights of its constituent elements. The space occupied by a compound is the volume that it takes up.

For example, consider the compound water (H2O). Water is composed of two hydrogen atoms and one oxygen atom. The weight of water is the sum of the weights of hydrogen and oxygen, which is 18 grams per mole. The volume of water is the space that it occupies, which is 18 milliliters per mole.