Ionic lattices, polarisation and metallic bonding

What you’ll learn

• Explain ionic and metallic properties using mobile particles and electrostatic attraction.
• Compare ionic radii and ionic-bond strength.
• Describe how polarisation introduces covalent character.

Understand the idea

Ionic bonding is the net electrostatic attraction among oppositely charged ions throughout a giant lattice. Electron transfer explains how ions may form, but is not itself the bond. Solid ionic compounds cannot conduct because their ions are fixed; molten or dissolved ions can move and carry charge.

Smaller separation and greater ionic charges generally strengthen lattice attraction. For isoelectronic ions, increasing proton number pulls the same number of electrons closer, decreasing radius. A small, highly charged cation strongly distorts an anion's electron cloud. A large anion is more easily polarised, giving greater covalent character.

In a metal, positive ion cores are attracted to delocalised electrons. Mobile electrons explain electrical conduction in both the solid and liquid. Metallic bonding is not confined to individual pairs of atoms, so layers can move while attraction remains; this helps explain malleability. Lattice models are supported by charge migration, electron-density evidence and the observed physical properties.

Key formulae

$$\mathrm{Mg\rightarrow Mg^{2+}+2e^-}$$

$$\mathrm{O+2e^-\rightarrow O^{2-}}$$

$$|F|\propto\frac{|q_1q_2|}{r^2}$$

$$r(\mathrm{Na^+})>r(\mathrm{Mg^{2+}})>r(\mathrm{Al^{3+}})$$

Worked example

Explain why magnesium oxide has a higher melting temperature than sodium chloride, using their ionic models.

1. Magnesium oxide contains $\mathrm{Mg^{2+}}$ and $\mathrm{O^{2-}}$; sodium chloride contains singly charged ions.
2. The magnesium oxide ions have greater charges and generally smaller separation.
3. Stronger electrostatic attractions require more energy to overcome when the solid melts.

Answer: The stronger ionic lattice attractions in magnesium oxide account for its higher melting temperature.