What is a compound?

Compounds are pure substances made of atoms of two or more elements chemically combined in fixed ratios. Compounds must meet all of these criteria:

Compounds are pure substances and not mixtures. For example, table salt is not a compound because it is a mixture of sodium chloride, dextrose and calcium silicate (to keep it from clumping in high humidity) and potassium iodide (an important nutrient necessary for proper thyroid function). But if these components are separated physically, each can be obtained in pure form and each meets all the criteria for compounds.
Compounds are made of atoms of two or more elements. For example, O₂ and O₃ are considered forms of the element oxygen, and not compounds of oxygen.
Compounds are chemically combined elements. Mixing 2.0 grams of H₂ with 70.9 g of Cl₂ in the dark results in a homogenous mixture of H₂ and Cl₂. Mix the gases in the exact same proportions in the light and there is a violent explosion. The gas is now 72.9 g of HCl, a compound. Components in a homogenous mixture may influence each other's chemical behavior slightly, but there is not radical alteration in chemical and physical properties. Compounds are completely different chemically and physically from the elements that form them.
Atoms of elements combine in fixed ratios to form compounds. Carbon dioxide is always composed of molecules with two atoms of oxygen for every one atom of carbon. Carbon monoxide has only one oxygen atom per atom of carbon- and it is a completely different substance than carbon monoxide.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! What is a compound?