Ad Banner Placeholder

Cambridge IGCSE Chemistry · 0620

Chapter 2: Atoms, elements and compounds — Part 3

Topic 2.5 · Simple molecules and covalent bonding

The covalent bond

A covalent bond is formed when two non-metal atoms share one or more pairs of electrons so that both atoms achieve a stable outer-shell electron arrangement (often a full outer shell of 8, like a noble gas).

Unlike ionic bonding, electrons are shared, not transferred. Covalent bonding typically occurs between non-metal atoms.

Covalent molecules

Covalent bonds can be single, double, or triple, depending on how many electron pairs are shared:

Single covalent bond
One shared pair of electrons. Examples: H2 (H—H), CH4 (four C—H bonds), Cl2.
Double covalent bond
Two shared pairs of electrons. Examples: O2 (O=O), CO2 (two C=O bonds).
Triple covalent bond
Three shared pairs of electrons. Example: N2 (N=N).

Methanol (CH3OH) is a covalent molecule containing single covalent bonds: three C—H bonds, one C—O bond, and one O—H bond.

Dot-and-cross diagrams for CO2, N2 and other simple covalent molecules
Diagram 1: Dot-and-cross diagrams for CO2, N2 and other simple molecules

Exam Traps

  • Do not draw ionic transfer in covalent molecules — O2 and N2 share electrons, they do not form ions.

Properties of simple molecular compounds

Low melting and boiling points
Simple molecular compounds have low melting and boiling points. The covalent bonds within molecules are strong, but the forces between molecules (intermolecular forces) are weak and require little energy to overcome.
Intermolecular forces
Attractions between separate molecules (not the bonds holding atoms together inside a molecule). These are much weaker than covalent, ionic, or metallic bonds.
Poor electrical conductivity
Simple molecular compounds are poor conductors of electricity in all states because they contain no mobile ions or free electrons — electrons are localised in shared pairs within molecules.

Exam Traps

  • Do not say covalent bonds are weak — it is the intermolecular forces that are weak and easy to overcome.
  • Do not claim simple molecules conduct because they contain electrons — those electrons are in fixed shared pairs, not free to move.

0/15

Ad Banner Placeholder