A cation is any atom or group of atoms that has a net positive charge. While matter is electrically neutral overall, ionic compounds are matter that is composed of positively-charged and negatively-charged particles called ions. An ion is any atom or group of atoms with an overall electrical charge. According to the laws of physics, opposite charges attract, so the oppositelycharged ions attract each other to form compounds that are, overall, electrically neutral. In such compounds, the number of positive charges on the cations is equal to the number of negative charges. Species that have an overall negative charge are called anions. Compounds that are composed of cations and anions are called ionic compounds. Examples include table salt (sodium chloride) and potash (potassium carbonate).
Cations are formed when an atom or group of atom loses one or more electrons. The resulting species has more protons than electrons, so it has an overall positive charge. (Each proton has a +1 charge, and each electron has a -1 charge. In normal atoms, the number of protons equals the number of electrons, so a normal atom has an overall charge of zero. We say it is electrically neutral.) On the other hand, anions are formed when an atom or group of atoms accepts one or more electrons, so it has more negative charges than positive charges. Most metallic elements react chemically to form cations, losing electrons. Most nonmetallic elements react chemically to gain electrons, thereby forming anions.
See also Anion.