Shapes of Molecules — Structured Questions (Cambridge International A-Level Chemistry): Exam Questions

Sulfur can form bonds with six fluorine atoms to form sulfur hexafluoride, SF₆.

i) State the number of electrons in the outer shell of the sulfur atom in SF₆.

ii) State the reason why the sulfur atom is able to accommodate this number of electrons.

State the shape of a molecule of SF₆.

State the F-S-F bond angles in SF₆.

Extract:

Sulfur can form bonds with six fluorine atoms to form sulfur hexafluoride, SF6.

The shape of a molecule of SF6: Octahedral

Sulfur has an electronegativity value of 2.58 and fluorine has an electronegativity value of 3.98.

i) Deduce whether the S-F bond is polar.

ii) Deduce whether the SF₆ molecule is polar. Explain your answer.

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°.

Extract:

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

{align=center} Fig. 1.1

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

Extract:

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

The bond angle in BH3 is 120° because:

• BH3 has 3 bonding pairs of electrons and no lone pairs
• The bonding pairs repel each other equally and arrange as far apart as possible to minimise repulsion

The H-N-H bond angle in NH3:

• 107°

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

ii) NBr₃

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.

Extract:

Methanamide, HCONH2, is the simplest aliphatic amide.

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.

Extract:

Methanamide, HCONH₂, is the simplest aliphatic amide.

The shapes around C and N in HCONH₂:

• Around C: trigonal planar
• Around N: trigonal pyramidal / pyramidal

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