For a given molecule, it electronic geometry equivalent to molecular geometry?

1 Answer
Mar 2, 2016

It can be, though sometimes it is the same as the molecular geometry.


I will try to illustrate with 2 examples: (i) the water molecule; and (ii) the methane molecule, #CH_4#. Around the carbon centre in methane there are 4 electron pairs, #4xxC-H# bonds. Valence shell electron pair theory predicts that these bonds would assume the shape of a tetrahedron (i.e. they want to be distributed according to the arrangement of least electron pair interaction), and indeed the molecular geometry is tetrahedral.

On the other hand, around the oxygen atom in the water molecular there are also 8 electrons: 4 in the #2xx# #O-H# bonds, and 4 as lone pairs centred on the oxygen atom (i.e. 2 lone pairs). Now VSEPR makes the same prediction for the water molecule: the bonding AND lone pairs should distribute in the arrangement of least interaction, so a tetrahedral geometry is predicted to a 1st approximation.

However, because we describe molecular geometry on the basis of atoms, and not on lone pairs, we describe the geometry of the water molecule as bent. To a second approximation we would predict that the lone pairs would compress the #H-O-H# bond angle from #109.5^@#, and an actual bond angle of #104.5^@# justifies this approximation.

If you have any queries or objections, please ask me to clarify. This is a fairly fundamental aspect of bonding and structure to get your head around.