VSEPR theory
From Biocrawler, the free encyclopedia.
The valence shell electron pair repulsion theory (VSEPR) is a theory in chemistry that aims to predict the shapes of molecules. The molecule is represented using the classical Lewis dot structure with bonding pairs and lone pairs of electrons.
The theory (as the name suggests) has its foundation in three types of repulsions taking place in these molecules between the electrons of the molecule:
- The lone pair-lone pair repulsion
- The lone pair-bonding pair repulsion between the atoms
- The bonding pair-bonding pair repulsion.
The molecule must avoid these repulsions to remain stable. The theory states that the repulsion becomes zero at ~115-120°. Sometimes, when repusion cannot be avoided, the less severe repulsion is preffered. The lone pair-lone pair (lp-lp) repulsion is said to exceed the lone pair-bonding pair (lp-bp) repulsion which is further said to exceed the bonding pair-bonding pair (bp-bp) repulsion. Hence, the lesser bp-bp repulsion is preffered over the lp-lp or lp-bp repulsion.
Larger molecules which fail to even maintain 90° between their electron pairs prefer to lie in more than one plane.
To predict the molecular geometry, the total coordination number of the central atom is taken into account.
The AXE method is commonly used in formatting molecules to fit the VSEPR model.
