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Lesson: Chapter 10 — Chemical Bonds

10.2. Covalent bonds 3 of 8

Water, ammonia and methane

When atoms of different elements share, the number of bonds an atom forms is set by how many electrons it is short of a full shell.

Water

Oxygen is 2, 6, so it is two electrons short. It shares a pair with each of two hydrogen atoms, forming two single bonds.

The water molecule: an oxygen atom sharing one pair of electrons with each of two hydrogen atoms, and keeping two pairs of its own.

Figure to be added

An oxygen atom and two hydrogen atoms drawn as overlapping shells, forming the water molecule (figure 10.10).

Ammonia

Nitrogen is 2, 5 and is three electrons short. Three hydrogen atoms supply three shared pairs, giving three single bonds.

The ammonia molecule: a nitrogen atom sharing one pair of electrons with each of three hydrogen atoms, and keeping one pair of its own.

Figure to be added

A nitrogen atom and three hydrogen atoms drawn as overlapping shells, forming the ammonia molecule (figure 10.11).

Methane

Carbon is 2, 4 and is four electrons short — the largest number in this group. Four hydrogen atoms give four shared pairs and four single bonds, and carbon has nothing left over.

The methane molecule: a carbon atom sharing one pair of electrons with each of four hydrogen atoms, with no pairs left over.

Figure to be added

A carbon atom and four hydrogen atoms drawn as overlapping shells, forming the methane molecule (figure 10.12).
Central atom Configuration Electrons short Bonds formed
Oxygen 2, 6 2 2
Nitrogen 2, 5 3 3
Carbon 2, 4 4 4

Count the gap to eight and you have counted the bonds. That one step predicts the formula of water, ammonia and methane without any of them being memorised.