Covalent bonding; shared pairs of electrons; small molecules; giant covalent structures such as diamond, graphite and silicon dioxide; and how structure explains properties.

What this dot point is asking

AQA wants you to explain how covalent bonds form by sharing electrons, draw dot and cross diagrams for small molecules, and contrast simple molecular substances with giant covalent structures such as diamond, graphite and silicon dioxide, explaining each property from the structure. The recurring exam skill is explaining a property (melting point, hardness, conductivity) by referring to the specific structure.

Forming covalent bonds

When non-metal atoms bond, they share pairs of electrons so each atom gains a full outer shell. Each shared pair is a single covalent bond, shown by a line or by overlapping electrons in a dot and cross diagram. Some atoms share more than one pair: oxygen forms a double bond ($O=O$) and nitrogen a triple bond ($N\equiv N$).

Small molecules

Substances such as $H_2O$, $CH_4$ and $Cl_2$ are simple molecules with low melting and boiling points, so many are liquids or gases at room temperature. They do not conduct electricity because the molecules are neutral overall with no free electrons or ions to carry charge. As molecules get larger, the intermolecular forces increase, so larger molecules have higher boiling points.

Giant covalent structures

• Diamond: each carbon forms four covalent bonds, giving a rigid 3D structure that is very hard and does not conduct (no free electrons).
• Graphite: each carbon forms three bonds in flat layers; the layers are held by weak forces and slide over each other (soft, slippery), and each carbon has one delocalised electron, so graphite conducts electricity.
• Silicon dioxide: a hard, very high melting point lattice similar to diamond, used in sand and glass.

Polymers

Polymers are very large molecules made of many repeating units joined by covalent bonds. They are usually solid at room temperature because the intermolecular forces between the long chains are relatively strong (much stronger than between small molecules), so more energy is needed to separate them.

Try this

Q1. Explain why simple molecular substances have low boiling points. [2 marks]

• Cue. Weak intermolecular forces between molecules need little energy to overcome.

Q2. Explain why graphite conducts electricity but diamond does not. [2 marks]

• Cue. Graphite has delocalised electrons that can move; diamond has none.

Q3. State why giant covalent structures have very high melting points. [1 mark]

• Cue. Many strong covalent bonds must be broken.