Simple molecules and covalent bonds
Sharing electron pairs, dot-and-cross diagrams, and the properties of simple molecular substances.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 2.5.1Describe the formation of a covalent bond as a shared pair of electrons between two non-metal atoms.
- 2.5.2Draw dot-and-cross diagrams for simple molecules including H₂, Cl₂, H₂O, CH₄, NH₃, CO₂ and N₂.
- 2.5.3Explain the low melting points and poor conductivity of simple molecular compounds in terms of weak intermolecular forces.Supplement
8 minute read
Covalent bonding
A covalent bond is a shared pair of electrons between two atoms, usually between two non-metals. Both atoms contribute one electron to the shared pair, and both count the shared pair towards a full outer shell — this is what holds them together and what gives each atom the stable electron arrangement of a noble gas.
Dot-and-cross diagrams
These show only the outer-shell electrons, using dots for one atom's electrons and crosses for the other's, so it is clear which atom each electron originally came from. You should be able to draw them for: H₂, Cl₂, H₂O, CH₄, NH₃, CO₂ and N₂.
- H₂: one shared pair between the two hydrogen atoms (single bond).
- CO₂: each oxygen shares two pairs with carbon (a double bond on each side).
- N₂: the two nitrogen atoms share three pairs (a triple bond), which is why nitrogen gas is so unreactive.
Properties of simple molecular substances
Substances made of small covalent molecules (like water, carbon dioxide, methane) typically have low melting and boiling points, and do not conduct electricity.
This is because:
- The covalent bonds within each molecule are strong, but boiling or melting does not break these.
- What actually gets overcome is the weak intermolecular forces between separate molecules — and these need only a small amount of energy, giving low melting and boiling points.
- There are no free ions or delocalised electrons available to carry charge, so these substances do not conduct electricity, whether solid, liquid, or (for most) even dissolved.
Think of it like this
A covalent molecule is like two people sharing one umbrella (the shared electron pair) — the umbrella bonds them together tightly. But the "crowd" of separate umbrella-sharing pairs standing near each other in the rain (the intermolecular forces) are only loosely gathered, and it takes very little effort to scatter the crowd, even though no umbrella itself needs to be broken.
Common misconceptions
- Believing a low boiling point means covalent bonds are weak. Covalent bonds themselves are strong; it is only the much weaker forces *between* separate molecules that are overcome on melting or boiling.
- Assuming any compound containing carbon or hydrogen must be a "simple molecule". Giant covalent structures like diamond also contain only covalent bonds, but behave completely differently because there are no separate molecules.
- Forgetting that a double or triple bond is still one bond overall, made of two or three shared pairs — not two or three separate bonds.
In the exam
- When explaining low melting points, name explicitly what breaks (weak intermolecular forces) and what does NOT break (strong covalent bonds) — examiners specifically reward this contrast.
- For dot-and-cross diagrams, always check every atom (except hydrogen) ends up with 8 electron dots/crosses around it (a full outer shell), and hydrogen ends up with 2.