What basic principle of physics governs VSEPR theory? What property of molecules is significantly affected by that principle?

1 Answer
Jan 11, 2017


#"VSEPR"# relies on electrostatic repulsion between like electronic charges.............


And of course the name implies this, #"valence shell electron pair repulsion theory."# And vesper allows us to predict the geometric shape of molecules, simply by counting up the number of valence electron pairs around a cental atom. The electron pairs, bonding or not, repel each other, and the electron pairs assume the shape that allows least electrostatic interaction, shapes that correspond to the Platonic solids.

I will discuss tetrahderal geometry in reference to 2 simple molecules. Methane has formula #CH_4#, there are 4 hydrogen electrons, and 4 valence carbon electrons. We have to distribute the bonding pairs of electrons around the carbon centre, and these pairs assume the shape of a tetrahedron, a four-sided figure.

The #/_H-C-H# bond angles are thus #109.5^@#, precisely those angles observed in a tetrahedron. Now ammonia has a formula of #NH_3#, but here there are also 8 electrons to distribute about the nitrogen: #3xxN-H# bonds, and one nitrogen-centred lone pair. The electronic geometry is STILL tetrahedral as before, however, we describe molecular shape on the basis of the disposition of atoms not on electronic geometry.

The lone pair on nitrogen is thus stereochemically active, and the #/_H-N-H# angles are compressed down from #109.5^@# to about #107^@# (of course this is found by experiment), to give a trigonal pyramidal molecular geometry. Given that lone pairs are close to the central atom, they tend to have a substantial geometric influence. Water has essentially the same geometry operating, but here there are TWO lone pairs, and the #/_H-O-H# angles are compressed down from #109.5^@# to about #104.5^@# due to the stereochemcial influence of the 2 oxygen centred lone pairs.

I urge you to read the relevant chapter in your text. I have said essentially the same thing in this earlier answer.