Describe covalent bonding as shared pairs of electrons, draw dot-and-cross diagrams for simple molecules, and relate simple molecular structure to low melting points and poor conduction.

What this topic is asking

WJEC topic 2.1 wants you to explain covalent bonding as the sharing of pairs of electrons between non-metal atoms, to draw dot-and-cross diagrams for simple molecules, and to relate the simple molecular structure to properties such as low melting points and not conducting electricity.

How a covalent bond forms

Non-metal atoms have nearly full outer shells. Rather than transferring electrons, two non-metal atoms share electrons so that each completes its outer shell. A single shared pair is a single covalent bond, drawn as a line between the atoms. Some atoms share more than one pair: oxygen molecules ($\text{O}_2$) have a double bond (two shared pairs), and nitrogen molecules ($\text{N}_2$) have a triple bond.

Dot-and-cross diagrams for molecules

A dot-and-cross diagram for a molecule shows the outer electrons, with one atom's electrons as dots and the other's as crosses, so the shared pairs are clear:

• Hydrogen, $\text{H}_2$: two hydrogen atoms share one pair, giving each a full first shell of 2.
• Chlorine, $\text{Cl}_2$: two chlorine atoms share one pair, each ending with 8 outer electrons.
• Water, $\text{H}_2\text{O}$: oxygen shares one pair with each hydrogen, giving two covalent bonds.
• Methane, $\text{CH}_4$: carbon shares one pair with each of four hydrogens, giving four covalent bonds.

Simple molecular substances

Many covalent substances exist as small, separate molecules - this is simple molecular structure. The key idea is that there are two kinds of force:

• strong covalent bonds holding the atoms together inside each molecule, and
• weak intermolecular forces between separate molecules.

Properties of simple molecular substances

As molecules get bigger, the intermolecular forces get stronger, so larger molecules have higher melting and boiling points.