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 covalent bonds
Between neighbouring molecules, there are weak intermolecular forces
When the substance melts or boils, only the weak intermolecular forces break; the strong covalent bonds inside the molecules stay intact
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
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