Ionic bonding

When a metal reacts with a non-metal, electrons move from the metal atoms to the non-metal atoms. The metal atoms become positive ions and the non-metal atoms become negative ions, and each ion has a full outer shell. The oppositely charged ions attract each other strongly in every direction, building a giant ionic lattice.

Key facts

  • Metal atoms lose their outer electrons to form positive ions; non-metal atoms gain electrons to form negative ions.
  • Group 1 metals form 1+ ions, group 2 form 2+ and aluminium 3+. Oxygen and sulfur form 2− ions; the halogens form 1− ions.
  • An ionic bond is the strong electrostatic attraction between oppositely charged ions.
  • In the formula of an ionic compound the charges cancel out: Mg²⁺ and Cl⁻ make MgCl2.
  • Dot-and-cross diagrams show the outer electrons: one atom’s electrons as dots and the other’s as crosses, with each ion in square brackets.
  • Ionic compounds have high melting points, and conduct electricity only when melted or dissolved in water, when their ions can move.

Worked examples

Example 1

Show how sodium and chlorine form sodium chloride, NaCl, with a dot-and-cross diagram.

  1. Sodium is 2,8,1: it has one outer electron. Chlorine is 2,8,7: it needs one more.
  2. Sodium gives its outer electron (the dot) to chlorine.
  3. Na⁺ is 2,8 and Cl⁻ is 2,8,8: both have full outer shells. Each ion goes in square brackets with its charge.

Answer: [Na]⁺ [Cl]⁻: sodium’s outer electron moves to chlorine

Example 2

Write the formula of aluminium oxide.

  1. Aluminium has 3 outer electrons, so it forms Al³⁺. Oxygen needs 2 more electrons, so it forms O²⁻.
  2. Find the lowest number both charges go into: 6.
  3. Two Al³⁺ ions give +6 and three O²⁻ ions give −6, so the charges cancel. 2 × 3 = 3 × 2 = 6
  4. So the formula is Al2O3.

Answer: Al2O3

Example 3

Magnesium oxide melts at 2852°C but sodium chloride melts at 801°C. Explain why.

  1. Both are giant ionic lattices. Melting means overcoming the attractions between all the ions.
  2. Magnesium oxide is made of Mg²⁺ and O²⁻ ions; sodium chloride of Na⁺ and Cl⁻ ions.
  3. Ions with bigger charges attract each other more strongly, so more energy is needed to melt magnesium oxide.

Answer: MgO is made of 2+ and 2− ions, which attract more strongly than the 1+ and 1− ions in NaCl