2) The theory could not explain why ClB^eCl in BeCl
2is 180°,FB^F inBF3
is 120°,HC^H in CH4is109°28',HN^H in NH3is107°48'and HO^H in H2O is104°31'etc.
i.e., it fails to explain the
shapes of the molecules.
| Bond | Bond length (A0) | Bond (dissociation) energy (KJmol-1) |
|---|---|---|
| H–H | 0.74 | 436 |
| F–F | 159 | |
| Cl–Cl | 1.95 | 243 |
| Br–Br | 2.28 | 193 |
| I–I | 2.68 | 151 |
| H–F | 0.918 | 570 |
| H–Cl | 1.27 | 432 |
| H–Br | 1.42 | 366 |
| H–I | 1.61 | 298 |
| H–O(of H2 O) | 0.96 | 460 |
| H–N(of NH3 ) | 1.01 | 390 |
| H–C (of CH4 ) | 1.10 | 410 |
To explain the bond angles in the molecules with three or more than three atoms with all atoms attached to a central atom through covalent bonds a theory called the valence – shell – electron – pair repulsion – theory (VSEPRT) was proposed by Sidgwick and Powell (1940). It was further improved by Gillespie and Nyholm (1957).
This theory suggests the following points.
1. VSEPRT considers electrons in the valence shells which are in covalent bonds and in lone pairs as charge clouds that repel one another and stay as far apart as possible. This is the reason why molecules get specific shapes.
2. If we know the total number of electron pairs in the valence shell as covalent bonds and lone pairs in the central atom, it will help us to predict the arrangement of those pairs around the nucleus of the central atom and from that the shape of the molecule.
3. Lone pairs occupy more space around the central atom than bond pairs. Lone pair means unshared electron pair or non-bonding electron pair. These are attracted to only one nucleus where as the bond pair is shared by two nuclei. Thus, the presence of lone pairs on the central atom causes slight distortion of the bond angles from the expected regular shape. If the angle between lone pair and bond pair increases at the central atom due to more repulsion, the actual bond angles between atoms must be decreased.