Two related topics that students seem to find difficult are ‘Shapes of simple molecules and ions” and’ Hybridization’. Shapes of simple molecules and ions is covered using VSEPR theory for 2, 3 and 4 pairs of electrons under sub-topic 4.3 - Covalent structures for Standard Level and extended to include 5 and 6 pairs in 14.1 at Higher Level. In addition shapes for sp, sp2 and sp3 hybridization are covered at Higher...