Metallic bonding

AS Level Chemistry · Chemical bonding. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

In a metal, each atom gives up its outer-shell electrons. The atoms become positive ions, packed closely in a regular giant lattice. The electrons that were given up are delocalised: they are free to move through the whole structure and do not belong to any one ion.

Metallic bonding is the electrostatic attraction between the positive metal ions and the delocalised electrons. It acts in all directions and is strong.

This one picture explains the properties of metals. Moving electrons carry charge. Layers of ions can slide while the electrons still hold them. Strong attraction needs a lot of energy to overcome.

Rules to remember

Common mistake

Students write that the metal ions move to carry the current, or call the bonding 'intermolecular forces'. Only the delocalised electrons move, and a metal contains no molecules.

Worked example

Calcium has a much higher melting point than potassium. Explain this using metallic bonding.

  1. Potassium forms K+ ions and gives one delocalised electron per atom.
  2. Calcium forms Ca2+ ions and gives two delocalised electrons per atom; Ca2+ is also smaller than K+.
  3. A smaller ion with a higher charge attracts more delocalised electrons more strongly.

Answer: The metallic bonding in calcium is stronger, so more energy is needed to melt it.

Practice questions

Try each one, then open the answer.

1. Why do the melting points of the Group 1 metals decrease from lithium to caesium?

  1. A
    the ions get larger with the same charge, so the attraction between the ions and the delocalised electrons weakens
  2. B
    the number of delocalised electrons per atom decreases down the group
  3. C
    the intermolecular forces between the atoms get weaker
  4. D
    the metals change from metallic to covalent bonding
Show answer

Answer: A. Each Group 1 atom supplies one delocalised electron. Down the group the ions have more shells, so the delocalised electrons are further from the nucleus and are attracted less strongly.

2. Why can solid copper conduct electricity?

  1. A
    the delocalised electrons are free to move through the lattice
  2. B
    the positive copper ions are free to move through the lattice
  3. C
    copper atoms are held together by weak intermolecular forces
  4. D
    both the ions and the electrons move towards the positive terminal
Show answer

Answer: A. In a metal the positive ions stay fixed in the lattice, but the delocalised electrons can move through it and carry charge when a voltage is applied.

3. Why are metals malleable?

  1. A
    the metallic bonds are weak and break easily
  2. B
    the delocalised electrons repel each other when the metal is hit
  3. C
    layers of positive ions can slide over each other while the delocalised electrons keep holding them together
  4. D
    the metal ions are held in fixed positions by directional bonds
Show answer

Answer: C. Metallic bonding is non-directional: when layers of ions slide, the delocalised electrons still attract them in their new positions, so the metal changes shape without breaking.

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