Q6(a)(i). Lewis structures and VSEPR shapes of SO₂, I₃⁻, SF₄, BrF₅, XeF₄

1. Lewis structure. Add the valence electrons of all atoms and the charge. Link the atoms by single bonds, complete the octet of each outer atom, and put the electrons left over on the central atom as lone pairs. Atoms of Period 3 and below can hold more than eight electrons (expanded octet).
2. VSEPR. The bond pairs and lone pairs on the central atom keep as far apart as possible. Their number fixes the arrangement; the positions of the atoms alone give the shape. A double bond counts as one domain.
3. Lone pairs are held by one nucleus only and spread more than bond pairs, so lp–bp repulsion is larger than bp–bp. In a trigonal bipyramid they take equatorial positions (two neighbours at 90° instead of three). In an octahedron two lone pairs go trans to each other.

SO₂. With two S=O bonds every formal charge is zero; the octet form O=S⁺-O⁻ with its resonance partner gives the same result. Both S–O bonds are equal (143 pm). The lone pair squeezes the angle slightly below 120°.

I₃⁻ and SF₄. In I₃⁻ the three lone pairs fill the equatorial plane and repel the two bonds equally, so the ion is exactly linear. In SF₄ the single lone pair is equatorial; it pushes the axial F atoms back to 173° and closes the equatorial angle from 120° to 101.6°.

BrF₅ and XeF₄. All six octahedral positions are equivalent, so one lone pair takes any one and the five F atoms form a square pyramid. The lone pair pushes the four basal F atoms up towards the apical F (angle below 90°; 81.9° in the analogous IF₅). In XeF₄ the two lone pairs are opposite each other (lp–lp at 180°), and the four F atoms lie in a square.

Application. the shape shows in the ¹⁹F NMR spectrum. Square-pyramidal BrF₅ has two kinds of F (4 basal + 1 apical): a doublet of intensity 4 and a quintet of intensity 1. A trigonal bipyramid would give 3 + 2 instead.