Ionic (Electrovalent) Bond

An ionic (or electrovalent) bond forms when one atom transfers one or more valence electrons completely to another atom, generating a positively charged ion (a cation) and a negatively charged ion (an anion). The oppositely charged ions are then held together purely by electrostatic (Coulombic) attraction — there is no sharing of electrons at all.

Ionic bond formation is favoured when one partner can lose electrons easily and the other can gain electrons readily and exothermically. Three quantities govern this:

1. Low ionisation enthalpy of the electropositive atom (usually a metal), so that removing an electron costs little energy.
2. High (negative) electron gain enthalpy of the electronegative atom (usually a non-metal), so that adding an electron releases energy.
3. High lattice enthalpy of the resulting ionic solid, which supplies the large amount of energy needed to overcome the ionisation enthalpy and to disperse the crystal into gaseous ions.

Consider the textbook example, sodium chloride. A sodium atom ($2, 8, 1$) loses its single valence electron to form $\text{Na}^+$ ($2, 8$), and a chlorine atom ($2, 8, 7$) gains that electron to form $\text{Cl}^-$ ($2, 8, 8$) — both ions now have a stable, noble-gas-like octet. The two oppositely charged ions attract each other electrostatically, and in the solid state this attraction extends indefinitely: each $\text{Na}^+$ ion surrounds itself with six $\text{Cl}^-$ ions and vice versa, building the extended three-dimensional rock-salt crystal lattice rather than a single isolated "molecule" of $\text{NaCl}$.

Lattice enthalpy ($\Delta_{lattice}H$) is defined as the energy released when one mole of an ionic solid is formed from its free gaseous ions (or, equivalently, the energy required to separate one mole of the ionic solid completely into gaseous ions). It is the single most important measure of how strongly an ionic lattice is held together, and by Coulomb's law it depends directly on the product of the ionic charges and inversely on the distance between the ion centres:

$U \propto \frac{Q_{+} \, Q_{-}}{r_{+} + r_{-}}$ …