Question

How does VSEPR work?

Answer 1

The key idea in `"VSEPR"` is that electron pairs, bonding and non-bonding, arrange themselves in the most electrostatically favourable orientation.

Explanation:

So if there are 2 electron pairs around a central atom, a linear molecule will result; if there are 3 electron pairs around a central atom, the electron pairs will arrange themselves in a trigonal plane; and if there are 4 electron pairs around a central atom, the electron pairs will assume a tetrahedral shape.

The important proviso, however, is that we do not describe molecular on the basis of electron pairs, but on the basis of bonds. Around carbon in methane there are 4 electron pairs in `4xxC-H` bonds, which assume the shape of tetrahedron; hence we describe the structure of methane as a tetrahedron.

On the other hand, around the oxygen atom in water there are also 4 electron pairs, `2xxO-H` bonds, and two oxygen lone pairs. Now the orientation of these electron pairs about oxygen ALSO assumes a tetrahedral shape, as the shape that minimizes interaction between the electron pairs. But we describe molecular geometry on the basis of bonding pairs, not lone pairs. The `/_H-O-H` bond is tetrahedral to a first approximation, but interaction from the lone pairs compress `/_H-O-H` to approx. `105^@` down from `109.5^@`, and the water molecule is described as bent.

Now the shapes of the electron pairs mirror that of the Platonic solids: 2 electron pairs, linear; 3 electron pairs, trigonal planar; 4 electron pairs, tetrahedral; 5 electron pairs, trigonal bipyramidal; 6 electron pairs, octahedral. Whether the molecules assume these shapes depends on the number of bonding and lone pairs, but it is fairly straightforward to predict molecular geometry on the basis of `"VESPER"`.

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