Metallic bonding

O Level / IGCSE Chemistry · Atoms, elements and compounds. 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 solid metal the atoms are packed closely in regular layers. Each atom gives up its outer shell electrons, so the structure is a giant lattice of positive ions. The electrons that have left the atoms are delocalised: they belong to no particular ion and move freely through the whole lattice, like a sea of electrons.

Metallic bonding is the electrostatic attraction between the positive ions and this sea of delocalised electrons.

This one model explains the main properties of metals. The moving electrons carry electric current. The layers of ions can slide over one another while the electrons keep holding them together, so a metal changes shape instead of breaking.

Rules to remember

Common mistake

Do not say a solid metal conducts because its ions move. The ions are fixed in the lattice; the delocalised electrons carry the current.

Worked example

A rod of copper is pulled through a small hole to make a long thin wire. The wire does not snap and it still conducts electricity. Explain both observations.

  1. Copper is a giant lattice of positive ions in layers, surrounded by a sea of delocalised electrons.
  2. When the rod is pulled, the layers of ions slide over one another into new positions.
  3. The delocalised electrons still attract the ions in their new positions, so the metallic bonding is not broken and the wire does not snap.
  4. The delocalised electrons are still free to move along the wire, so it still conducts.

Answer: Copper is ductile because its layers of ions can slide while the sea of electrons holds them together, and it conducts because those electrons are free to move.

Practice questions

Try each one, then open the answer.

1. Solid copper conducts electricity but solid copper(II) chloride does not. Why?

  1. A
    copper has positive ions that are free to move through the solid
  2. B
    copper(II) chloride is made of neutral molecules with no charges
  3. C
    copper has mobile delocalised electrons; solid copper(II) chloride has fixed ions
  4. D
    copper(II) chloride has delocalised electrons that cannot move
Show answer

Answer: C. A metal conducts through its mobile delocalised electrons. An ionic solid has no mobile charged particles until it is melted or dissolved.

2. A small piece of solid sodium contains 6.0 × 1023 sodium ions, Na+. How many delocalised electrons does it contain, and what is its overall charge?

  1. A
    1.2 × 1024 electrons; overall neutral
  2. B
    6.0 × 1023 electrons; overall charge 6.0 × 1023+
  3. C
    3.0 × 1023 electrons; overall neutral
  4. D
    6.0 × 1023 electrons; overall neutral
Show answer

Answer: D. Each sodium atom gives one electron to the sea of delocalised electrons, so the number of electrons equals the number of Na+ ions. The positive and negative charges balance, so the metal is neutral.

3. Aluminium (proton number 13) has a metallic lattice. How many delocalised electrons does each atom contribute to the lattice, and what is the charge on each metal ion?

  1. A
    1 electron; 1+
  2. B
    3 electrons; 3+
  3. C
    3 electrons; 3−
  4. D
    13 electrons; 13+
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Answer: B. An aluminium atom is 2,8,3. It releases its three outer electrons into the sea of delocalised electrons, leaving an Al3+ ion. The inner shells stay in the ion.

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