S2.3 The metallic modelIB Chemistry SL: Revision notes

Metal atoms release their outer electrons, which become delocalised — shared by the whole structure rather than belonging to one atom. The result is a regular lattice of cations in a sea of delocalised electrons.

A metallic bond is the electrostatic attraction between a lattice of cations and delocalised electrons. It acts in all directions, so it is non-directional. The number of delocalised electrons per atom usually equals the number of outer electrons: one for sodium, two for magnesium, three for aluminium.

Electrical conductivity: when a potential difference is applied, the delocalised electrons drift towards the positive terminal, carrying charge. The cations stay in fixed lattice positions. Metals conduct when solid and when molten.

Thermal conductivity: delocalised electrons gain kinetic energy where the metal is hot and move quickly through the lattice, carrying energy to cooler regions. Closely packed cations also pass vibrations to their neighbours.

More delocalised electrons per atom generally means more charge carriers — aluminium (three per atom) conducts better than sodium (one per atom).

Metals are malleable (can be hammered into sheets) and ductile (can be drawn into wires). When a force is applied, layers of cations slide over one another. Because the bonding is non-directional, the delocalised electrons continue to attract the cations in their new positions, so the metal changes shape instead of breaking.

Compare ionic solids: sliding a layer brings ions of the same charge next to each other, they repel and the crystal shatters.

The attraction is stronger when:

• the charge of the cation is higher (and more electrons are delocalised per atom);
• the radius of the cation is smaller, so the delocalised electrons are closer to the centre of positive charge.

Both together are described as charge density. A stronger metallic bond means more energy is needed to separate the particles, so the melting point is higher.

Down group 1 (Li 181 °C → Cs 28 °C): the charge stays 1+ but the ionic radius increases, so the attraction to the delocalised electrons weakens and the melting point falls. The same pattern appears down group 2.

Across period 3 (Na 98 °C, Mg 650 °C, Al 660 °C): the cation charge rises (1+, 2+, 3+), the radius falls and more electrons are delocalised, so metallic bonding strengthens and the melting point rises.

Charge and radius must be considered together: Na⁺ (102 pm) and Ca²⁺ (100 pm) are almost the same size, but calcium melts at 842 °C because of its higher charge.