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IGCSE · Practice questions · Simple molecules and covalent bonds

Count the shared pairs.

Six original Cambridge-style questions on covalent bonding: what a covalent bond is, dot-and-cross diagrams for ammonia, ethene and methanol, and why simple molecules melt easily and do not conduct.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Every atom ends full.

01
Recall
[2 marks]

State what is meant by a covalent bond and explain why atoms form one.

  • A covalent bond is a pair of electrons shared between two atoms. ✓
  • By sharing, each atom gains a full outer shell (noble-gas configuration). ✓
02
Analysis
[3 marks]

Describe the dot-and-cross diagram for ammonia, NH3. State the number of bonding pairs and lone pairs and the number of outer electrons around nitrogen.

  • Three shared pairs, each made of one nitrogen electron and one hydrogen electron (three N-H bonds). ✓
  • One lone pair on the nitrogen atom. ✓
  • Nitrogen has 3 × 2 + 2 = 8 outer electrons; each hydrogen has 2. ✓
03
Analysis
[3 marks]

(Extended) For ethene, C2H4, state the number of shared pairs of electrons in the molecule and the total number of bonding electrons. Show that each carbon atom has eight outer electrons.

  • One C=C double bond (2 shared pairs) and four C-H bonds (4 shared pairs): 6 shared pairs. ✓
  • Total bonding electrons = 6 × 2 = 12. ✓
  • Each carbon: 4 electrons in the double bond + 2 × 2 in its two C-H bonds = 8. ✓
04
Analysis
[2 marks]

(Extended) Methanol has the formula CH3OH. State the number of single covalent bonds in the molecule and the number of lone pairs on the oxygen atom.

  • Five single bonds: three C-H, one C-O and one O-H. ✓
  • Two lone pairs on the oxygen atom. ✓
05
Explain
[3 marks]

(Extended) Chlorine, Cl2, is a gas at room temperature. Explain why, in terms of structure and bonding.

  • Chlorine is made of small simple molecules. ✓
  • The intermolecular forces between the molecules are weak, so little energy is needed to overcome them. ✓
  • The strong covalent bond within each Cl2 molecule is not broken when chlorine boils. ✓
06
Explain
[2 marks]

Explain why simple molecular substances such as methane are poor conductors of electricity.

  • The molecules have no overall charge. ✓
  • There are no ions or delocalised electrons free to move and carry charge. ✓

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.