Ionic compounds typically form when a metal atom transfers one or more electrons to a nonmetal atom. This creates a stable cation and a stable anion, each with a full outer electron shell. Polyatomic ions, like ammonium (NH4+) or sulfate (SO4 2-), can also pair with an oppositely charged ion to form ionic compounds.
In the solid state, ions stay locked in fixed positions within the lattice. Breaking apart this lattice takes a lot of energy, since many strong ionic bonds must be overcome at once. This is why ionic compounds tend to be hard and brittle, with high melting and boiling points.
Solid ionic compounds do not conduct electricity, because the charged ions cannot move freely. Once melted or dissolved in water, though, the ions separate and become mobile, letting the resulting liquid or solution carry an electric current. The strength of an ionic bond depends on the charges of the ions and the distance between them, a relationship described by Coulomb’s law.
Ions with higher charges, or ions that pack closer together, form stronger bonds and generally higher melting points. Ionic compounds also tend to dissolve well in polar solvents like water. Polar water molecules can surround and separate individual ions from the crystal lattice.