A negatively charged ion, which has more electrons in its electron shells than it has protons in its nuclei, is known as an anion (ἀνά ana: Greek 'up') ( ; an-eye-on). Conversely, a positively-charged ion, which has fewer electrons than protons, is known as a cation (κατά kata: Greek 'down') ( ; cat-eye-on).

An ion consisting of a single atom is called a monatomic ion, but if it consists of two or more atoms, it is a polyatomic ion. Polyatomic ions containing oxygen, such as carbonate and sulfate, are called oxyanions.

Ions are denoted in the same way as electrically neutral atoms and molecules except for the presence of a superscript indicating the sign of the net electric charge and the number of electrons lost or gained, if more than one. For example: H⁺ and SO₄²⁻.

Formation of polyatomic and molecular ions

Polyatomic and molecular ions are often formed by the combination of elemental ions such as H⁺ with neutral molecules or by the gain of such elemental ions from neutral molecules. A simple example of this is the ammonium ion NH₄⁺ which can be formed by ammonia NH₃ accepting a proton, H⁺. Ammonia and ammonium have the same number of electrons in essentially the same electronic configuration but differ in protons. The charge has been added by the addition of a proton (H⁺) not the addition or removal of electrons. The distinction between this and the removal of an electron from the whole molecule is important in large systems because it usually results in much more stable ions with complete electron shells. For example NH₃·⁺ is not stable because of an incomplete valence shell around nitrogen and is in fact a radical ion.

Ionization potential

main: Ionization potential The energy required to detach an electron in its lowest energy state from an atom or molecule of a gas with less net electric charge is called the ionization potential, or ionization energy. The nth ionization energy of an atom is the energy required to detach its nth electron after the first n − 1 electrons have already been detached.

Each successive ionization energy is markedly greater than the last. Particularly great increases occur after any given block of atomic orbitals is exhausted of electrons. For this reason, ions tend to form in ways that leave them with full orbital blocks. For example, sodium has one valence electron, in its outermost shell, so in ionized form it is commonly found with one lost electron, as Na⁺. On the other side of the periodic table, chlorine has seven valence electrons, so in ionized form it is commonly found with one gained electron, as Cl⁻. Francium has the lowest ionization energy of all the elements and fluorine has the greatest. The ionization energy of metals is generally much lower than the ionization energy of nonmetals, which is why metals will generally lose electrons to form positively-charged ions while nonmetals will generally gain electrons to form negatively-charged ions.