4CH1

Covalent Bonding

Principles of Chemistry · 5 question types

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 8% of your exam marks.

stable
Low
Stable8%

Dot-and-cross diagrams and comparison of giant covalent vs simple molecular structures appear regularly.

Bonding inside vs forces between molecules

  • A simple molecular substance is built from many small discrete molecules (e.g. H₂O, O₂, CO₂, HCl, NH₃, CH₄)
  • Inside each molecule, the atoms are held by strong
  • Between neighbouring molecules, there are weak
  • When the substance melts or boils, only the weak intermolecular forces are overcome; the strong covalent bonds inside the molecules stay intact
Two water molecules showing strong covalent bonds within each molecule and weak intermolecular forces between the molecules
Source: Simple molecular structures by Save My Exams

Low melting and boiling points

  • Simple molecular substances generally have low melting and boiling points because:
    • The intermolecular forces are weak
    • Only a small amount of thermal energy is needed to overcome them
  • Most simple molecular substances are gases or liquids at room temperature; only a few exist as soft solids with low melting points
Exam tip

Low melting point of a simple molecular substance

What comes up: explain why a simple molecular substance melts or boils easily.

Watch out: the credited idea is that only the weak intermolecular forces between molecules are overcome — never write "the covalent bonds break" or "weak covalent bonds". The strong covalent bonds inside the molecules stay intact.

Trend: larger molecules, higher melting and boiling points

  • As the relative molecular mass of the molecule increases, melting and boiling points increase too
  • A bigger molecule has more electrons, so the intermolecular forces between molecules are stronger and need more thermal energy to overcome
  • e.g. in the alkane family (Topic 16), methane is a gas, hexane is a liquid, and the waxy long-chain alkanes are soft solids. The trend follows molecular size

Electrical conductivity

  • Simple molecular substances do not conduct electricity in any state (solid, liquid or gas)
  • Reason: there are no free charge carriers. No mobile ions and no delocalised electrons
  • This is why plastics, rubber, wax and most solid sugars are good electrical insulators — the plastic coating around household wiring is a covalent polymer for exactly this reason