Describe covalent bonding as the sharing of electrons, draw dot-and-cross diagrams for simple molecules, and relate simple molecular structures to their properties

What this dot point is asking

The syllabus wants you to describe covalent bonding as the sharing of pairs of electrons between non-metal atoms, to draw dot-and-cross diagrams for simple molecules such as hydrogen, water, methane, and carbon dioxide, and to link the simple molecular structure to its properties, low melting and boiling points and not conducting electricity. The key idea is that non-metals reach full outer shells by sharing electrons rather than transferring them.

The answer

How covalent bonds form

When two non-metal atoms bond, neither one gives up electrons. Instead they share a pair of electrons, so that both atoms count the shared pair toward a full outer shell. A shared pair of electrons is a covalent bond. Sharing two pairs gives a double bond, as in carbon dioxide and oxygen.

Dot-and-cross diagrams

A dot-and-cross diagram shows the outer-shell electrons of each atom, using dots for one atom and crosses for the other, with the shared pair sitting between the bonded atoms. Worked examples:

• Hydrogen, $\text{H}_2$: each hydrogen has one electron; they share one pair, giving each a full first shell of two.
• Water, $\text{H}_2\text{O}$: oxygen shares one pair with each of two hydrogens, so oxygen reaches eight and each hydrogen reaches two.
• Methane, $\text{CH}_4$: carbon shares one pair with each of four hydrogens, giving carbon eight outer electrons.
• Carbon dioxide, $\text{CO}_2$: carbon shares two pairs (a double bond) with each oxygen.

Simple molecular structures and their properties

A simple molecular substance is made of small separate molecules. The bonds inside each molecule are strong, but the forces between the molecules are weak. This explains:

• Low melting and boiling points: only the weak forces between molecules need to be overcome, so little energy is needed. Many are liquids or gases at room temperature.
• Do not conduct electricity: the molecules are neutral with no free ions or electrons to carry charge.
• Often insoluble in water (though some, like sugar, dissolve).

Examples in context

Example 1. Why oxygen and nitrogen are gases. The air is mostly oxygen and nitrogen, both simple molecular substances ($\text{O}_2$ and $\text{N}_2$). The forces between their molecules are so weak that they are gases at room temperature, even though the bonds inside each molecule are strong. This is typical of small covalent molecules.

Example 2. Why candle wax melts so easily. Candle wax is made of simple covalent molecules held together by weak forces. A small flame gives enough energy to overcome those weak forces, so the wax melts at a low temperature, while the covalent bonds inside the molecules remain unbroken until the wax burns.

Try this

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

• Cue. A covalent bond is a shared pair of electrons between two atoms (non-metals).

Q2. Explain why carbon forms four covalent bonds in methane. [2 marks]

• Cue. Carbon has four outer electrons and needs four more to reach a full outer shell of eight, so it shares one pair with each of four hydrogen atoms.

Q3. Explain why a simple molecular substance such as iodine has a low melting point. [2 marks]

• Cue. It is made of separate molecules held together by weak forces between molecules, so only a little energy is needed to separate them; the strong covalent bonds inside the molecules do not break on melting.