Covalent bonding as the sharing of electron pairs between non-metal atoms, drawing dot-and-cross diagrams for simple molecules, and the properties of simple molecular substances.

What this dot point is asking

CCEA wants you to explain covalent bonding as the sharing of electron pairs between non-metals, draw dot-and-cross diagrams for simple molecules, and use the idea of weak forces between molecules to explain the properties of simple molecular substances.

How covalent bonds form

Non-metals do not transfer electrons (both would want to gain), so instead they share. In hydrogen ($\text{H}_2$) each atom shares its one electron, so both reach the full shell of two. In oxygen ($\text{O}_2$) the atoms share two pairs, a double bond, so both reach eight.

Dot-and-cross diagrams for molecules

A dot-and-cross diagram shows the shared pairs. Use dots for one atom's electrons and crosses for the other's; a shared pair has one of each. Common molecules:

• Methane ($\text{CH}_4$): carbon shares with four hydrogens, four single bonds.
• Ammonia ($\text{NH}_3$): nitrogen shares with three hydrogens, with one lone pair left on nitrogen.
• Water ($\text{H}_2\text{O}$): oxygen shares with two hydrogens, with two lone pairs left on oxygen.

Show only the outer electrons, and check every atom ends with a full outer shell.

Properties of simple molecular substances

This explains the properties:

• Low melting and boiling points. Only weak intermolecular forces need to be overcome, so little energy is required; many simple molecular substances are gases or liquids at room temperature.
• Do not conduct electricity. The molecules are neutral, with no free electrons or ions to carry charge.
• Often insoluble in water unless the molecule can interact with water (such as ammonia).

Worked example

Examples in context

Example 1. Why oxygen and nitrogen are gases. The air is mostly small molecules of $\text{O}_2$ and $\text{N}_2$ with very weak forces between them, so they stay gaseous at room temperature. Their physical state is a direct result of being simple molecular substances.

Example 2. Carbon dioxide as a refrigerant. Solid carbon dioxide (dry ice) sublimes straight to gas at -78 degrees C because only weak intermolecular forces hold the molecules in the solid. This low sublimation point, useful for keeping things cold without liquid, follows from its simple molecular structure.

Try this

Q1. State what is meant by a covalent bond. [1 mark]

• Cue. A shared pair of electrons between two non-metal atoms.

Q2. Explain why methane has a low boiling point. [2 marks]

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