How does vsepr theory classify molecules?

1 Answer
Nov 2, 2015

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.


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 .