Q3(d). Hybridisation and shapes of SO₃ and PF₅

Reading: "SO₃⁻" is printed; the question speaks of "these molecules", so it is read as sulfur trioxide, SO₃. The sulfite ion SO₃²⁻ is added in the last point.

1. SO₃: S has 6 valence electrons, all used in bonding to three O atoms. Three σ bonds and no lone pair: steric number 3, so S is sp² hybridised.
2. Shape of SO₃: the three sp² orbitals point to the corners of an equilateral triangle: SO₃: trigonal planar, O–S–O=120° The π bonding (S 3p_z with O 2p) is delocalised over the three equivalent S–O bonds; the molecule is non-polar.
3. PF₅: P has 5 valence electrons, one shared with each F. Five σ bonds, no lone pair: steric number 5, so P uses one 3s, three 3p and one 3d_(z²) orbital: sp³d hybridisation.
4. Shape of PF₅: five pairs are farthest apart in a trigonal bipyramid: PF₅: trigonal bipyramidal, 120° (equatorial), 90° (axial–equatorial) Each axial F has three neighbours at 90°, each equatorial F only two, so the axial bonds are longer than the equatorial ones.
5. Pseudorotation: in solution the axial and equatorial F atoms exchange rapidly (Berry pseudorotation), so the ¹⁹F NMR spectrum shows a single environment.
6. If the ion was intended: sulfite, SO₃²⁻, has a lone pair on S (steric number 4): sp³, trigonal pyramidal, O–S–O a little below 109.5°. The radical SO₃˙⁻ (one non-bonding electron on S) is also pyramidal.

Application. SO₃ is the planar, strongly Lewis-acidic intermediate of the contact process (2SO₂ + O₂ → 2SO₃ on V₂O₅); its empty π* orbital accepts the lone pair of water or of H₂SO₄ (oleum).