Chemical Bonding (Ionic, Covalent and Metallic): Question 7

Worked solution

Part (a): Bonding in water

Oxygen and hydrogen are both non-metals, so they bond by sharing electrons rather than transferring them. This is covalent bonding.

Oxygen has 6 electrons in its outer shell and needs 2 more to reach a stable arrangement of 8 (like neon). Each hydrogen atom has 1 electron and needs 1 more to reach a stable arrangement of 2 (like helium). Oxygen therefore forms two separate covalent bonds, one with each hydrogen atom:

$\text{O (6 outer electrons)} + 2\,\text{H (1 outer electron each)} \rightarrow \text{H}_2\text{O}$

In each O–H bond, one electron comes from oxygen and one comes from hydrogen, and this pair of electrons is shared between the two atoms. After bonding, oxygen has 8 electrons around it (2 shared pairs $+$ 2 unshared lone pairs), and each hydrogen has 2 electrons around it (its own electron $+$ 1 shared from oxygen), both reaching stable, noble-gas-like electron arrangements.

Part (b): Why water has a comparatively low boiling point

Water is a simple molecular substance: it exists as small, separate $\text{H}_2\text{O}$ molecules, not a giant lattice of ions or atoms. Boiling water means separating these molecules from one another, which only requires overcoming the weak forces of attraction between molecules, the strong covalent O–H bonds within each molecule stay intact.

By contrast, melting an ionic compound such as sodium chloride means breaking apart a giant lattice held together by strong electrostatic forces between oppositely charged ions throughout the whole structure, which needs far more energy. This is why water’s boiling point ($100\text{ °C}$) is so much lower than sodium chloride’s melting point ($801\text{ °C}$), even though the covalent bonds inside a water molecule are themselves strong.

Part (c): Electrical conductivity of pure water

Pure water does not conduct electricity. It consists of neutral $\text{H}_2\text{O}$ molecules with no free-moving ions and no delocalised electrons, so there is nothing available to carry a charge through the liquid.

Final answers

• (a) Covalent bonding: 2 shared pairs of electrons, one per O–H bond; oxygen ends with 8 outer electrons (2 bonding pairs $+$ 2 lone pairs), each hydrogen ends with 2.
• (b) Water is simple molecular; only weak forces between molecules are broken on boiling, not the strong covalent bonds within, so its boiling point is far lower than an ionic lattice’s melting point.
• (c) Pure water does not conduct electricity, no free ions or delocalised electrons.