How do lewis structures help predict the shape of a compund?

1 Answer
Oct 28, 2015

The correct Lewis structure around an atom allows us to note the number of shared and lone electron pairs around a particular atom. These pairs of electrons arrange themselves in the shapes of the Platonic solids in the molecule.


Lewis structure is taught in conjunction with VSEPR, Valence shell electron pair repulsion theory. To a first approximation, it doesn't matter whether the electron pairs are lone pairs or bonding pairs. They will arrange themselves in order to minimize electron-electron repulsion between electron pairs. Two electron pairs (around the central atom) will be 180#@# with respect to each other; three electron pairs, 120#@#; four electrons pairs, and the overall structure becomes 3-dimensional, tetrahedral, and the electron pairs (bonding or not) are disposed 109.5#@# with respect to each other; 5 electron pairs, trigonal bipyramidal; and 6 electron pairs, octahedral.

All of this treatment relies on the Lewis structure. For a given set of atoms, we know the number of bonding electrons from the Periodic table. For main group structures, VSEPR allows us to make accurate predictions as to the structure of the molecule.