Ionic 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

Ionic bonding is the electrostatic attraction between oppositely charged ions: positive cations and negative anions. The ions form when a metal atom transfers its outer electrons to a non-metal atom. Each ion usually ends up with the electronic configuration of a noble gas.

The ions are not in pairs. They pack into a giant ionic lattice, in which each ion attracts all the oppositely charged ions around it, in every direction. There are no molecules.

The formula shows the ratio of ions that makes the charges cancel. Smaller ions and ions with higher charges attract more strongly, so the melting point is higher.

Rules to remember

Common mistake

Students describe ionic bonding as the transfer of electrons. The transfer makes the ions; the bond is the attraction between the oppositely charged ions afterwards.

Worked example

Potassium (proton number 19) reacts with sulfur (proton number 16). What is the formula of the compound, and what configuration does each ion have?

  1. K is 2,8,8,1 and loses one electron to form K+ (2,8,8).
  2. S is 2,8,6 and gains two electrons to form S2− (2,8,8).
  3. Two K+ are needed to balance one S2−: (2 × +1) + (−2) = 0.

Answer: K2S; both ions have the electronic configuration of argon.

Practice questions

Try each one, then open the answer.

1. In calcium fluoride, which noble gas electronic configurations do the ions have?

  1. A
    both like argon
  2. B
    both like neon
  3. C
    Ca2+ like neon; F− like argon
  4. D
    Ca2+ like argon; F− like neon
Show answer

Answer: D. Calcium (2,8,8,2) loses two electrons to become 2,8,8, like argon; fluorine (2,7) gains one electron to become 2,8, like neon.

2. Ionic bonding is best described as

  1. A
    the sharing of a pair of electrons between two atoms
  2. B
    the electrostatic attraction between oppositely charged ions
  3. C
    the attraction between positive ions and delocalised electrons
  4. D
    the attraction between molecules with permanent dipoles
Show answer

Answer: B. Ionic bonding is the electrostatic attraction between cations and anions in a giant lattice. Shared pairs describe covalent bonding; ions plus delocalised electrons describe metallic bonding.

3. Aluminium and oxygen form an ionic compound. What are its formula and the ions it contains?

  1. A
    Al2O3, containing Al3+ and O2−
  2. B
    AlO, containing Al2+ and O2−
  3. C
    Al3O2, containing Al2+ and O3−
  4. D
    AlO3, containing Al3+ and O−
Show answer

Answer: A. Aluminium (Group 13) forms Al3+ and oxygen (Group 16) forms O2−. Two Al3+ (+6) balance three O2− (−6), and the oppositely charged ions attract electrostatically.

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