Metallic bonding

In a metal the atoms give up their outer electrons. This leaves a regular lattice of positive ions in a “sea” of delocalised electrons that move freely through the whole structure. The strong attraction between the positive ions and the delocalised electrons is metallic bonding, and it explains why metals conduct, bend without breaking and have high melting points.

Key facts

  • Metallic bonding is the strong attraction between positive metal ions and delocalised electrons.
  • Each atom gives its outer electrons to the sea: sodium gives 1, magnesium 2 and aluminium 3.
  • Delocalised electrons can move and carry charge and energy, so metals conduct electricity and heat.
  • The layers of ions can slide over each other, so pure metals can be hammered into shape and drawn into wires.
  • Most metals have high melting points, because the metallic bonding is strong throughout a giant structure.
  • An alloy is a mixture of a metal with other elements. Atoms of different sizes distort the layers so they cannot slide easily, so alloys are harder than pure metals.

Worked examples

Example 1

Describe the bonding in magnesium.

  1. Magnesium is 2,8,2: each atom has 2 outer electrons.
  2. Each atom gives these 2 electrons to a sea of delocalised electrons, which move through the whole metal.
  3. That leaves a regular lattice of Mg²⁺ ions. The strong attraction between the ions and the electrons holds the metal together.

Answer: a lattice of Mg²⁺ ions in a sea of delocalised electrons, 2 from each atom

Example 2

Explain why aluminium (melting point 660°C) has a higher melting point than sodium (98°C).

  1. Sodium gives 1 electron per atom and forms Na⁺ ions. Aluminium gives 3 per atom and forms Al³⁺ ions.
  2. Aluminium has more delocalised electrons and ions with a bigger charge, so the attraction between them is stronger.
  3. More energy is needed to overcome it, so aluminium melts at a higher temperature.

Answer: aluminium has 3+ ions and 3 delocalised electrons per atom, so its metallic bonding is stronger

Example 3

18-carat gold is 18/24 gold by mass. Find the mass of gold in a 6 g ring, and explain why jewellery is not made of pure gold.

  1. 18 carat means 18 parts out of 24 are gold.
  2. Find that fraction of the mass. 18 ÷ 24 × 6 = 4.5 g
  3. Pure gold is soft because its layers slide easily. The other metal atoms are a different size and distort the layers, so the alloy is harder and lasts longer.

Answer: 4.5 g; pure gold is too soft, and the alloy is harder