Question

How does vsepr theory classify molecules?

Answer 1

Valence shell electron pair repulsion theory (vesper for short) classifies molecules on the basis of the number of electron pairs, bonding and lone pairs, associated with the central atom.

Explanation:

It is a fact that electron pairs arranged around a central atom repel each other (hence vesper). In the molecule, they assume a shape, a geometry, that will minimize electronic interaction. These shapes can correspond to the Platonic solids: 2 pairs, linear; three pairs, trigonal planar; 4 pairs, tetrahedral; 5 pairs, trigonal pyramidal; 6 pairs, octahedral.

Sometimes, this geometry is not readily apparent. There are 4 electron pairs around the central oxygen atom in water, `OH_2`, (`8e` in total, 2 bonding `O-H` pairs, 2 lone pairs per oxygen atom), and to a first approximation these 4 electron pairs assume the shape of a tetrahedron, which has bond angles of `109.5^(@)`.

So the ideal `/_H-O-H` would be this value, however, as the two lone (or non-bonding) pairs are larger and more diffuse than bonding pairs, they tend to reduce/compress this `/_H-O-H` to `104-5^(@)`. But while the electron pairs assume this tetrahedral shape, molecular geometry is described in terms of the disposition of the actual atoms ; water is thus described as a bent molecule. How would I describe the geometry of the presumed acidium species in water, `H_3O^+`; is this the same geometry as in ammonia?

Also see this answer .