Ionic molecules consist of multiple atoms that have an electron number different from that of their ground state. When a metal atom bonds with a nonmetal atom, the metal atom typically loses an electron to the nonmetal atom. This is called an ionic bond. That this happens with compounds of metals and non-metals is a result of two periodic properties: ionization energy and electron affinity.

The metals of the periodic table include all the elements in groups one through three except hydrogen, as well as some other elements from the lower right-hand regions of the table. The nonmetals, on the other hand, include all the elements in groups seven and eight, as well as some other elements from groups four, five and six.

The ionization energy of an element describes the amount of energy needed to cause an atom to lose an electron. Metals tend to have low ionization energies. This means that they are "willing" to get rid of an electron in a chemical reaction. Many nonmetals, on the other hand, have high ionization energies, which means that they are less willing to lose an electron in a reaction.

Electron affinity is the change in energy when a neutral atom of an element gains an electron. Some atoms are more willing to gain electrons than others. Metals have a small electron affinity, and therefore do not willingly accept electrons. Many nonmetals, on the other hand, have large electron affinities; they release a larger amount of energy upon accepting electrons. This means that the nonmetals are far more willing to accept electrons than the metals are. This corresponds to their positions on the periodic table. The reactive nonmetals are close to group eight elements, which have full outermost electron shells. The group eight elements are very stable. Hence, a nonmetal that is one or two electrons away from a full electron shell will be eager to gain those electrons and reach a stable state.

The concepts of ionization energy and electron affinity are combined into a third periodic trend called electronegativity. Electronegativity differences between elements describe the type of bonds between the atoms. If electronegativity differences are very small, the bonds are covalent. If electronegativity differences are large, the bonds are ionic. The electronegativity differences between metals and most nonmetals are high. Therefore, the bonds have an ionic character. This makes sense with respect to ionization energy and electron affinity; the metal atoms are willing to lose electrons, and the nonmetal atoms are willing to gain them.