The metallic model

Chemistry · Structure 2: Models of bonding and structure. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

In a metal the atoms lose their outer electrons, which become delocalized: they are free to move through the whole structure and do not belong to any one atom. What is left is a regular lattice of positive ions (cations). Metallic bonding is the electrostatic attraction between this lattice of cations and the sea of delocalized electrons.

The bond is not directional. It acts in all directions, so layers of cations can slide over each other without the bond breaking. This is why metals are malleable.

The moving electrons carry charge and pass on kinetic energy, so metals conduct electricity and heat. The bonding is stronger when the cations are small and highly charged.

Rules to remember

Metallic bond: electrostatic attraction between a lattice of cations and delocalized electrons
Stronger bonding with a higher cation charge, more delocalized electrons per atom and a smaller cation radius
Electrical conductivity: delocalized electrons move when a potential difference is applied (the cations do not move)
Thermal conductivity: delocalized electrons pass kinetic energy quickly through the lattice
Across period 3, melting point rises: Na < Mg < Al (ions 1+, 2+, 3+ and getting smaller)
Down group 1, melting point falls: the ions get larger, so the attraction for the delocalized electrons is weaker

Common mistake

Saying that the ions move to carry the current in a solid metal. Only the delocalized electrons move; the cations stay in the lattice.

Worked example

Potassium and calcium are next to each other in period 4. Which has the higher melting point, and why?

Potassium forms K⁺ and gives one electron per atom to the delocalized sea; calcium forms Ca²⁺ and gives two.
Ca²⁺ is also smaller than K⁺.
A higher charge, more delocalized electrons and a smaller radius give a stronger attraction, so more energy is needed to melt calcium.

Answer: Calcium (842 °C, compared with 63 °C for potassium)

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Structure 2.3 The Metallic Model

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