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AS · Practice questions · Covalent and dative covalent bonding

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Six original Cambridge-style questions on covalent bonding: definitions, multiple bonds, dative bonds in familiar ions and molecules, and molecules that break the octet rule.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
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A shared pair holds two nuclei together.

01
Recall
[2 marks]

Define (a) a covalent bond and (b) a dative covalent bond.

  • (a) The electrostatic attraction between two nuclei and a shared pair of electrons. ✓
  • (b) A covalent bond in which both electrons of the shared pair are supplied by one of the atoms. ✓
02
Explain
[3 marks]

Describe how the ammonium ion forms from ammonia and a hydrogen ion, and explain why all four N–H bonds in the ion are the same length.

  • The nitrogen in NH3 has a lone pair; H+ has an empty 1s orbital. ✓
  • The lone pair is donated to H+, forming a dative covalent bond. ✓
  • Once formed, the dative bond is a shared pair like the others, so all four N–H bonds are identical. ✓
03
Explain
[3 marks]

At lower temperatures aluminium chloride vapour contains Al2Cl6 molecules. Explain how two AlCl3 units join together.

  • Each Al in AlCl3 has only six outer electrons, so it has an empty orbital. ✓
  • A chlorine atom in each AlCl3 has lone pairs. ✓
  • A lone pair from a Cl of one unit forms a dative bond to the Al of the other, and vice versa: two dative bonds. ✓
04
Recall
[2 marks]

State the number of bonding pairs and lone pairs around the central atom in (a) PCl5 and (b) BF3.

  • (a) PCl5: 5 bonding pairs, 0 lone pairs (10 electrons). ✓
  • (b) BF3: 3 bonding pairs, 0 lone pairs (6 electrons). ✓
05
Explain
[2 marks]

Explain why phosphorus can form PCl5 but nitrogen cannot form NCl5.

  • Phosphorus is in Period 3 and has empty 3d orbitals, so it can hold ten electrons in its outer shell. ✓
  • Nitrogen is in Period 2; its second shell can hold at most eight electrons, so it cannot form five bonds. ✓
06
Analysis
[2 marks]

Carbon monoxide contains a triple bond. Explain how the triple bond is made up, given that carbon has four and oxygen six outer electrons.

  • Two of the pairs are ordinary covalent bonds, one electron from C and one from O in each. ✓
  • The third pair is a dative bond: a lone pair on O is donated to C, so both atoms reach eight outer electrons. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.