5. Ionic Compounds Held By Ionic Bonds

In chemistry, an ionic compound is a chemical compound composed of ions held together by strong ionic bonds. The compound is neutral overall, but consists of positively charged ions called cations and negatively charged ions called anions. Compounds of metals and non-metals are usually ionic. They are made when two or more atoms come close together and an electron goes from one atom to the other atom. The electron does this because both atoms want a full outer shell, similar to those of the noble gases.

Ionic compounds containing basic ions hydroxide (OH⁻) or oxide (O²⁻) are classified as bases. Ionic compounds without these ions are also known as salts and can be formed by acid–base reactions. Ionic compounds can also be produced from their constituent ions by evaporation of their solvent, precipitation, freezing, a solid-state reaction, or the electron transfer reaction of reactive metals with reactive non-metals, such as halogen gases.

Lots of energy (heat) is required to break the bond apart, resulting in high melting and boiling points. Ionic compounds typically have high melting and boiling points, and are hard and brittle. Individual ions within an ionic compound usually have multiple nearest neighbors, so are not considered to be part of molecules, but instead part of a continuous three-dimensional network, usually in a crystalline structure. As solids they are almost always electrically insulating, but when melted or dissolved they become highly conductive, because the ions are mobilized.

Ions in ionic compounds are primarily held together by the electrostatic forces (the electric force between charged bodies at rest)  between the charge distribution of these bodies, that is these ionic bonds resulting from the long-ranged Coulomb attraction (the amount of force between two stationary, electrically charged particles) between the net negative charge of the anions and net positive charge of the cations. There is also a small additional attractive force from van der Waals interactions (a distance-dependent interaction between atoms or molecules) which contributes only around 1–2% of the cohesive energy for small ions. When a pair of ions comes close enough for their outer electron shells to overlap, a short-ranged repulsive force occurs, due to the Pauli exclusion principle. The balance between these forces leads to a potential energy well with a minimum energy when the nuclei are separated by a specific equilibrium distance.