All the substances which occur in nature consist of one or more basic elements; a substance containing more than one element is known as a compound. An element is a substance that can neither be decomposed (broken into a number of other substances) by ordinary chemical action, or made by a chemical union of a number of other substances. A compound consists of two or more different elements in combination and has properties different from the properties of its constituent parts. Water, for example, is a compound of oxygen and hydrogen . A molecule is the smallest amount of a substance that can retain the characteristic properties of that substance and may consist, for example, of two atoms of hydrogen and two atoms of oxygen for hydrogen peroxide, of one atom of oxygen and one atom of carbon for carbon monoxide and of two atoms of oxygen and one atom of carbon for carbon dioxide. An atom is the smallest unit of which a chemical element is built. The atoms of any particular element all have the same average mass and this average mass differs from the average mass of the atoms of any other element.