compound

Compounds whose molecules contain the benzene ring, or variations of it, are called aromatic. The term was originally used to distinguish sweet-smelling compounds from others.

Chemical substance made up of two or more elements bonded together, so that they cannot be separated by physical means. Compounds are held together by ionic or covalent bonds.

The name of a compound may give a clue to its composition. If the name ends in -ide (with the notable exceptions of hydroxides and ammonium chloride) it usually contain two elements. For example calcium oxide is a compound of calcium and oxygen.

If the name ends in -ate or -ite the compound contains oxygen; compounds ending in -ate have a greater proportion of oxygen than those ending in -ite. For example, sodium sulphate (Na₂SO₄) contains more oxygen than does sodium sulphite (Na₂SO₃).

If the name starts with per- the compound contains extra oxygen. For example, hydrogen peroxide H₂O₂ contains one more oxygen than hydrogen oxide (water) H₂O.

The prefix thio- indicates that the compound contains an atom of sulphur in place of oxygen. For example, sodium thiosulphate (Na₂S₂O₃) contains one more sulphur and one less oxygen than the more common sodium sulphate (Na₂SO₄).

The proportions of the different elements in a compound are shown by the chemical formula of that compound. For example, a molecule of sodium sulphate, represented by the formula Na₂SO₄ contains two atoms of sodium, one of sulphur, and four of oxygen.