Name the theory responsible for the definite geometry of covalent molecules. Give main features of the theory ?
The theory responsible for the definite geometry of covalent molecules is called valence shell electron pair repulsion theory (VSEPR theory).
The main points of this theory are:
1. The geometry of a molecule depends upon the number of electron pairs (bonded pairs as well as non bonded) surrounding the central atom.
2. The electron pairs surrounding the central atom repel each other because their electron clouds are negatively charged. As a result, electron pairs try to stay as far away as possible to attain a state of minimum enthalpy or maximum stability.
3. The relative magnitudes of the relative interactions between pair of electrons on the central atom are as follows:
Lone pair-Lone pair (lp | lp) > Lone pair-Bond pair (lp | bp) > Bond pair-Bond pair (bp | bp).
4. Repulsive forces decrease sharply with increasing angle between the electron pairs. They are strong at 90°, weaker at 120° and much weaker at 180°.
Different geometries of molecules depending upon the number of electron pairs in the valence shell of the central atom are summed up as:
| No. of electron pairs around central atom | Geometric arrangement | Bond angles | Examples |
| 1 | Linear | 180° | BeF2, ZnCl2, HgCl2 |
| 2 | Trigonal Planar | 120° | BF3, AlCl3 |
| 3 | Tetrahedral | 109.5° | CH4,CCl4 |
| 4 | Trigonal | 120° | SiCl4 |
| 5 | Bipyramidal | 90° | PF5, PCl5 |
| 6 | Octahedral | 90° | SF6, TeF6 |
