Q.(a) Explain VSEPR theory. Applying this theory to predict the shapes of IF7 and SF6.

This choice question asks either for the VSEPR theory with the shapes of IF7 and SF6, or for completing two reactions and drawing three structures. …

This is a 5-mark either/or question; both alternatives are answered below.

PART (a) — VSEPR theory and shapes of IF7 and SF6:

VSEPR (Valence Shell Electron Pair Repulsion) theory states that the electron pairs (bonding and lone pairs) in the valence shell of the central atom repel one another and arrange themselves in space so as to be as far apart as possible, minimising repulsion; this arrangement decides the molecular shape. The order of repulsion strength is: lone pair-lone pair > lone pair-bond pair > bond pair-bond pair. Lone pairs therefore distort ideal bond angles.

• IF7: iodine is the central atom bonded to 7 fluorine atoms, with 7 bond pairs and no lone pairs. Seven electron domains give sp3d3 hybridisation and a pentagonal bipyramidal shape (five F in a pentagon in the equatorial plane, two F axial).
• SF6: sulphur is bonded to 6 fluorine atoms, with 6 bond pairs and no lone pairs. Six electron domains give sp3d2 hybridisation and a regular octahedral shape (all F-S-F angles 90°).

PART (b)(i) — Completing the reactions:

• CH3Br + KOH(aq) → CH3OH + KBr (nucleophilic substitution: OH- replaces Br- to give methanol). …