Question

Shell model for NH 3 (H atomic #1, mass#1, N atomic #7, mass#14

Answer 1

Valence Shell Electron Pair Repulsion theory (VSEPR) is a set of rules whereby the chemist may predict the shape of an isolated molecule. It is based on the premise that groups of electrons surrounding a central atom repel each other, and that to minimize the overall energy of the molecule, these groups of electrons try to get as far apart as possible. Groups of electrons can refer to electrons that participate in a bond (single, double, or triple) to another atom, or to non-bonding electrons (e.g. lone pair electrons).

The ideal electronic symmetry of a molecule consisting of a central atom surrounded by a number of substituents (bonded atoms and non-bonding electrons) is characteristic of the total number of substituents, and is determined solely by geometric considerations the substituents are arranged so as to maximize the distances amongst them. VSEPR is useful for predicting the shape of a molecule when there are between 2 and 6 substituents around the central atom.

Ammonia also has four electron pairs and the coordination geometry of nitrogen is based upon a tetrahedral arrangement of electron pairs. There are just three bonded groups, therefore there is one lone pair. However since the lone pairs are 'invisible', the shape of ammonia is pyramidal.

Consider a bonding pair of electrons. The two electrons are located between two nuclei, and are attracted by both. A lone pair is different. It is necessarily only attracted to one nucleus and the consequence is that it adopts a position effectively closer to that one nucleus than the bonding pairs of electrons. This means that the effective solid angle occupied by a lone pair is greater than that occupied by a bond pair. Lone pairs demand greater angular room, and are located closer to their atoms than bond pairs. The consequence of this for ammonia is that the lone pair makes room for itself by pushing the three hydrogen atoms together a little and the H-N-H bond angles are slightly less (106.6