Question

What leads to a divergence between the real geometry and the ideal geometry predicted by vsepr theory?

Answer 1

 Molecules comprised of main group elements adopt the structure that minimizes the repulsions between bond pairs (bp) of e⁻s and lone pairs (lp) of e⁻s (VSEPR theory: Wikipedia). However lp are only attracted to one +ve nucleus and consequently are more diffuse (spread out) and hence in terms of e⁻↔ e⁻ repulsion: lp-lp>lp-bp>bp-bp. So we have: CH4 AX4 regular tetrahedron angles all 109.5°; NH3 trigonal pyramidal AX3E the lone pair squeezes the H-N-H angles to <109.5° (~108°); for H2O: AX2E2 V shaped or bent - now we have lp-lp repulsion the H-O-H closes to significantly less than 109.5° (~104°). 
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