(a)

(i) Cross and dot diagram:

(ii) NH₃ molecules contain a lone pair of electrons.

(iii) Shapes: Linear and V-shaped

(iv)

In the CH₄ molecule the centre of the positive charge will be at the geometric centre of the molecule so it coincides with the centre of negative charge; therefore, this molecule is non-polar.

In the BF₃ molecule the centre of the negative charge is at the geometric centre of the molecule so it coincides with the centre of positive charge; therefore, this molecule is non-polar.

(v) The bond angle for NH₃ (107.0) is less than the bond angle for CH₄ (109.5). The presence of the lone pair on the NH₃ molecule causes the bonding pairs to be pushed closer together and this causes a decrease in the bond angle.