Compound contains two or more simpler compounds that can be packed in a definite ratio into a crystal.
A dot is used to separate the compounds in the formula.
For example, ZnSO4·7H2O is an addition compound of zinc sulphate and water. This represents a compound, and not a mixture, because there is a definite 1:7 ratio of zinc sulphate to water in the compound.
Hydrates are a common type of addition compound.
See also: Compound.