Distinguishing elements, compounds and mixtures

Substances can be distinguished by the types of atoms they contain and whether those atoms are chemically bonded. An element contains one type of atom. A compound contains different elements chemically bonded together. A mixture contains substances present together without being chemically bonded to one another.

Elements and compounds

An element is a pure substance made of only one type of atom. It cannot be broken down into simpler substances by chemical means. An element can contain molecules: oxygen gas, for example, contains $$\ce{O2}$$ molecules, but each molecule contains only oxygen atoms.

A compound is a pure substance in which atoms of two or more different elements are chemically bonded in fixed proportions. For example, water is a compound: each $$\ce{H2O}$$ molecule contains hydrogen and oxygen atoms bonded in a fixed ratio of two hydrogen atoms to one oxygen atom. Separating the elements in a compound requires a chemical change that breaks their bonds.

Mixtures

A mixture contains two or more substances that are not chemically bonded to one another. The proportions of those substances can vary, and each substance retains its own properties. The components can be separated by physical methods, without breaking chemical bonds between the substances.

For example, iron and sulfur can be mixed in different proportions. The iron retains its magnetic property, so a magnet can separate it from the sulfur. This differs from a compound, in which the elements are chemically bonded in fixed proportions.