(a)(1) Define covalent bond. (ii) Give two properties of covalent compounds (ii) With the aid of a diagram, show how ammonia molecule is formed (iv) Illustr...

(a) Covalent bonding

(i) A covalent bond is a bond formed when two atoms contribute and share one or more pairs of electrons.

(ii) Properties of covalent compounds include:

• They are generally non-conductors of electricity.
• They usually have low melting and boiling points.
• They are usually insoluble in water but soluble in non-polar organic solvents.

(iii) and (iv) The dot-and-cross diagrams below show the formation of ammonia and ammonium ion. Dots represent electrons from nitrogen, while crosses represent electrons from hydrogen.

(v)

1. In ammonia, NH₃, the N-H bonds are ordinary covalent bonds.
2. In ammonium ion, NH₄⁺, there are covalent bonds and one co-ordinate (dative covalent) bond formed by donation of the lone pair on nitrogen to H⁺. After formation, the four N-H bonds are equivalent.

(b)

(i) The subatomic particles and their relative masses are:

(ii) Water exists as a solid (ice), liquid (water), and gas (steam or water vapour).

(c) Diffusion

(i) Graham's law states that, at constant temperature and pressure, the rate of diffusion of a gas is inversely proportional to the square root of its molar mass or vapour density.

\[r \propto \frac{1}{\sqrt{M}}\]

(ii) \(M_r(\mathrm{He})=4\), \(M_r(\mathrm{CH_4})=16\), and \(M_r(\mathrm{N_2})=28\). Hence, the order of increasing rate of diffusion is:

\[\boxed{\mathrm{N_2\; <\; CH_4\; <\; He}}\]

This is because gases with smaller molar masses diffuse faster.

(d) Structural formulae