Chemical Bonding and Molecular Structure
Chemical Bonding and Molecular Structure tests whether you can move from a Lewis structure to shape, hybridisation, polarity and, for simple diatomics, bond order.
Core rules
- Formal charge = (valence electrons) − (non-bonding electrons) − ½(bonding electrons). Pick the structure whose formal charges sit closest to zero.
- VSEPR counts electron domains: a lone pair, a single bond, a double bond and a triple bond each count as one domain.
- Repulsion order lp–lp > lp–bp > bp–bp, which is why H₂O is bent at about 104.5° and NH₃ pyramidal at about 107°.
- Hybridisation from domain number: 2 → sp (linear), 3 → sp² (trigonal planar), 4 → sp³ (tetrahedral), 5 → trigonal bipyramidal with lone pairs equatorial, 6 → octahedral.
- Molecular-orbital bond order = (bonding − antibonding)/2.
Worked example
Why is O₂ paramagnetic? Fill the diagram: σ2s² σ2s² σ2p² π2p⁴ π2p². Bond order = (8 − 4)/2 = 2, a double bond, consistent with the short O–O distance. The two π* electrons occupy separate orbitals with parallel spins (Hund), giving two unpaired electrons and therefore paramagnetism. Removing one electron to form O₂⁺ raises the bond order to 2.5 and shortens the bond.
Traps
- Counting a double bond as two VSEPR domains; it counts as one.
- Reporting electron geometry when the question asks for molecular shape: water is tetrahedral electronically but bent molecularly.
- Assuming polar bonds always give a polar molecule: the two polar C=O bonds in CO₂ cancel by symmetry.