Shapes of Molecules (CIE AS Chemistry)

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°.

Predict whether the bond angle in NH₃ is greater or smaller than the bond angle in BH₃.

Explain why the bond angle in NH₃ is different from the bond angle in BH_3.

The Valence Shell Electron Pair Repulsion Theory (VSEPR) is used to predict the shapes of many chemical molecules.

Describe the main features of the VSEPR theory for predicting shapes of molecules.

State and explain the bond angle F-O-F in OF₂.

Predict and explain the shapes and bond angles of the following molecules:

i)        BF₃ 

[2]

ii)       NBr₃ 

[2]

Ethene, C₂H₄, and hydrazine, N₂H₄, are hydrides of adjacent elements in the periodic table. 

State and explain the H一C一H bond angle in ethene and the H一N一H bond angle in hydrazine.

Hydrazine can be oxidised to form diimide, which is a useful compound used in organic synthesis. 

Deduce the molecular geometry of diimide, N₂H₂, and estimate its H–N–N bond angle.

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

Predict the value of the H–N–H bond angle in N₂H₆²⁺.

Amides are commonly used in industrial processes as solvents, and there are suggestions that the smallest amide, methanamide, HCONH₂, could function in a similar manner to water as a solvent for life on other planets.

Draw the dot-and-cross diagram for methanamide, HCONH₂.

Predict and explain the H-C-N and C-N-H bond angles around the C and N atoms.

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

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