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Chemical Bonding (Ionic, Covalent and Metallic): Question 6

Syllabus 2.4

Multiple choice Extended 1 mark

Aluminium oxide is a hard, chemically stable compound that forms a thin, protective coating naturally on aluminium cooking foil and window frames, stopping the metal underneath from corroding further. Aluminium atoms (Group III) react with oxygen atoms (Group VI) to form this compound. Which statement correctly describes the ions formed and the ratio in which they combine in aluminium oxide?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Identify the group of each element and its outer-shell electrons

Aluminium is in Group III, so an aluminium atom has 3 electrons in its outer shell. Oxygen is in Group VI, so an oxygen atom has 6 electrons in its outer shell.

Step 2: Work out the ions formed

To reach a stable, noble-gas-like electron arrangement, each aluminium atom loses all 3 of its outer electrons:

Al→Al3++3e−\text{Al} \rightarrow \text{Al}^{3+} + 3e^-

Each oxygen atom needs just 2 more electrons to complete its outer shell, so it gains 2 electrons:

O+2e−→O2−\text{O} + 2e^- \rightarrow \text{O}^{2-}

Step 3: Balance the charges

An Al3+\text{Al}^{3+} ion carries 3 units of positive charge, and an O2−\text{O}^{2-} ion carries 2 units of negative charge. For the overall compound to be electrically neutral, the total positive charge must equal the total negative charge. The lowest common multiple of 3 and 2 is 6, so 2 aluminium ions (total charge +6+6) balance 3 oxide ions (total charge −6-6). This gives the formula Al2O3\text{Al}_2\text{O}_3.

Why the other options are wrong

  • B: aluminium is in Group III, so it loses 3 electrons, not 2. An Al2+\text{Al}^{2+} ion is incorrect.
  • C: the ion charges are correct, but a 1:11:1 ratio does not balance the charge. One Al3+\text{Al}^{3+} and one O2−\text{O}^{2-} would leave a net charge of +1+1, which is not possible for a neutral compound.
  • D: aluminium (a metal) and oxygen (a non-metal) form an ionic bond by electron transfer, not a covalent bond by electron sharing.

Final answer

The correct statement is A\boxed{\text{A}}: aluminium transfers 3 electrons per atom to oxygen, forming Al3+\text{Al}^{3+} and O2−\text{O}^{2-} ions combined in a 2:32:3 ratio, giving Al2O3\text{Al}_2\text{O}_3.