Ions and ionic bonds

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

An ion is an atom that has lost or gained electrons to get a full outer shell. Metal atoms lose their outer electrons and form positive ions, called cations. Non-metal atoms gain electrons and form negative ions, called anions.

When a metal reacts with a non-metal, electrons are transferred from the metal atoms to the non-metal atoms. The oppositely charged ions then attract each other strongly. This attraction is the ionic bond.

The ions pack together in a giant lattice, a regular arrangement of alternating positive and negative ions. There are no separate molecules. The strong attractions throughout the lattice explain the high melting points of ionic compounds.

Rules to remember

Common mistake

In a molten or dissolved ionic compound the current is carried by moving ions, not by electrons. Writing "free electrons" here loses the mark.

Worked example

Calcium (2,8,8,2) reacts with fluorine (2,7). What ions are formed and what is the formula of calcium fluoride?

  1. A calcium atom loses its 2 outer electrons to become Ca2+, with configuration 2,8,8.
  2. A fluorine atom gains 1 electron to become F−, with configuration 2,8.
  3. One calcium atom gives away 2 electrons, so 2 fluorine atoms are needed to take them: charges (2+) + 2 × (1−) = 0.

Answer: The ions are Ca2+ and F−; the formula is CaF2.

Practice questions

Try each one, then open the answer.

1. An ionic bond is

  1. A
    the attraction between positive ions and delocalised electrons
  2. B
    a shared pair of electrons between two non-metal atoms
  3. C
    a strong electrostatic attraction between oppositely charged ions
  4. D
    a weak force of attraction between separate simple molecules
Show answer

Answer: C. Ionic bonding is the strong electrostatic attraction between positive and negative ions in a giant lattice.

2. Element Z forms an ionic compound with nitrogen with the formula Z3N2. The nitride ion is N3−. In which group of the Periodic Table is Z?

  1. A
    Group I
  2. B
    Group II
  3. C
    Group III
  4. D
    Group V
Show answer

Answer: B. The charges must balance: 2 × 3− = 6−, so three Z ions must carry 6+ in total, which is 2+ each. An element forming 2+ ions is in Group II.

3. Which set of properties is most likely to belong to an ionic compound?

  1. A
    melting point 770 °C; conducts when molten or dissolved but not when solid
  2. B
    melting point 1710 °C; insoluble; does not conduct in any state
  3. C
    melting point 1083 °C; conducts when solid and when molten
  4. D
    melting point −95 °C; does not conduct in any state
Show answer

Answer: A. Ionic compounds have high melting points and conduct only when the ions are free to move (molten or aqueous). Conducting when solid suggests a metal; the others are molecular or giant covalent.

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