Shapes of Molecules (Cambridge (CIE) A Level Chemistry): Exam Questions

The shapes of the molecules BH₃ and NH₃ are shown in Fig. 1.1.

Fig. 1.1

Explain why each bond angle in BH₃ is 120°.

State the H-N-H bond angle in a molecule of NH₃.

Explain this difference in bond angles.

State the basic principles of the Valence Shell Electron Pair Repulsion (VSEPR) theory used to predict the shapes of molecules.

State the shape of, and the F-O-F bond angle in, a molecule of OF₂. Explain your answer.

State the shape of, and the bond angle in, each of the following molecules.

i) BF₃

[2]

ii) NBr₃

[2]

State and explain the H-C-H bond angle in ethene, C₂H₄, and the H-N-H bond angle in hydrazine, N₂H₄.

Hydrazine can be oxidised to form diimide, HN=NH.

State the shape around each nitrogen atom in diimide and predict the H-N-N bond angle.

Hydrazine forms a cation with an ethane-like structure called hydrazinediium, N₂H₆²⁺.

Predict the H-N-H bond angle in the N₂H₆²⁺ cation.

Methanamide, HCONH₂, is the simplest aliphatic amide.

Draw a dot-and-cross diagram to show the bonding in a molecule of methanamide.

Show outer shell electrons only.

State and explain the H-C-N and C-N-H bond angles around the C and N atoms in HCONH₂.

State the shape around each of the C and N atoms in HCONH₂.

State, with a reason, whether HCONH₂ is a polar molecule.