Ionic bonding

Definition. Ionic bonding is the electrostatic attraction between oppositely charged ions — positive cations and negative anions.

Ionic compounds form between a metal (which loses electrons) and a non-metal (which gains electrons), where the electronegativity difference is large.

The bond is not between a single pair of ions only — in a solid, each ion is attracted to all the oppositely charged ions surrounding it, throughout a giant lattice.

Atoms transfer electrons to achieve a stable noble-gas configuration.

Sodium chloride, NaCl. Na ($2,8,1$) loses 1 electron → $\text{Na}^+$ ($2,8$); Cl ($2,8,7$) gains it → $\text{Cl}^-$ ($2,8,8$). $\text{Na} \rightarrow \text{Na}^+ + e^- \qquad \text{Cl} + e^- \rightarrow \text{Cl}^-$ Ratio 1 : 1.

Magnesium oxide, MgO. Mg ($2,8,2$) loses 2 electrons → $\text{Mg}^{2+}$; O ($2,6$) gains 2 → $\text{O}^{2-}$. Ratio 1 : 1, but with 2+ and 2− charges.

Calcium fluoride, CaF₂. Ca ($2,8,8,2$) loses 2 electrons → $\text{Ca}^{2+}$; each F gains 1 → $\text{F}^-$. One Ca²⁺ transfers to two F atoms → ratio 1 : 2.

In a dot-and-cross diagram (Topic 3.7), the metal's electrons are shown moving into the non-metal's outer shell, and the ions are drawn in square brackets with their charge.

Ionic compounds exist as giant ionic lattices: a regular, repeating 3D arrangement of alternating cations and anions, held together by strong electrostatic forces in all directions.

Strength of ionic bonding (and therefore melting point) increases with:

• Greater ionic charge — e.g. $\text{Mg}^{2+}$ and $\text{O}^{2-}$ attract far more strongly than $\text{Na}^+$ and $\text{Cl}^-$.
• Smaller ionic radius — smaller ions sit closer together, so the attraction is stronger.

Worked comparison. MgO melts at ~2850 °C but NaCl at ~801 °C. MgO has 2+ and 2− ions (versus 1+ and 1−) and smaller ions, so the electrostatic attraction is much stronger → far more energy is needed to break the lattice.

(The detailed effect of charge and radius on lattice energy is developed at A Level in 23.1. Physical properties of ionic solids — high melting point, conduction only when molten/aqueous, brittleness, solubility — are covered in 4.2 States of matter.)