Lone pairs (Chemistry Question)...

Question

Dear experts,

I would like to know whether the lone pairs of the atoms actually (squeeze) the atom into forming

specific molecular structures. For example PH3 and NH3 molecules both have 1 lone pair, is that

the reason why these molecules form a trigonal pyramidal and not a trigonal planar shape?

Answer 1

The molecular shape of a covalently bonded molecule or the shape of a polyatomic ion is based on the VSEPR Theory (Valence Shell Electron Pair Repulsion Theory) to try to explain the three dimensional geometry of a molecule which is based on the supposition that electron pairs will orient themselves in such a way as to minimize the repulsive affects that the electrons have, given electrons have the same charge. The lone pairs repel other pairs of electrons and the molecule will geometrically arrange itself in space to minimize this repulsion and help determine the molecular shape that is most advantageous.

Answer 2

VSEPR explains the distortion well. The reason a lone pair distorts the geometry is that when pairs of electrons are shared between two atoms the electrons in the bond are constricted to a tighter space. When there is a lone pair, the lack of a nucleus on the other side allows the electrons to roam more, which pushes the other electron pairs further away.