Ionic bonding involves the attraction between oppositely charged ions formed through a redox reaction. Sodium loses its outer electron to become a cation while that electron enters the fluorine atom exothermically to create an anion. The resulting electrostatic attraction forms solid sodium fluoride.
Atoms with low ionization energy donate electrons to those with high electron affinity to establish ionic bonds. Weakly electronegative alkali metals lose valence electrons easily to strongly electronegative halogens. This transfer creates cations from electron donors and anions from electron acceptors.
Ionic compounds form lattice structures where ions occupy the corners of the crystal unit cell. In sodium chloride each ion has six bonds with all bond angles at ninety degrees. Pauling's rules provide guidelines for predicting these specific crystal geometries based on coordination numbers.
The experimental value for lattice energy can be determined using the Born-Haber cycle method. It also calculates as the sum of electrostatic potential energy and a short-range repulsive potential energy term via the Born-Landé equation. For example the calculated value for sodium chloride is negative seven hundred fifty-six kilojoules per mole compared to negative seven hundred eighty-seven kilojoules per mole from the Born-Haber cycle.
Fajans' rules apply when a small positive ion distorts the electron cloud of a large negative ion. This polarization leads to extra charge density between nuclei resulting in partial covalency. The effect becomes important with three plus charges like aluminum three plus but also occurs with small one plus lithium ions.