Chemical substance

• A chemical compound is a substance consisting of two or more chemical elements that are chemically combined in fixed proportions.

• A chemical element is a substance that cannot be divided or changed into different substances by ordinary chemical methods. The smallest particle of such an element is an atom, which consists of electrons centered around a nucleus of protons and neutrons.

• A molecule is the smallest particle of an element or compound that retains the characteristics of the element or compound.

• An ion is an atom or group of atoms with a net electric charge, having lost (cation) or gained (anion) an electron.