Geometry or Shape of Molecules:
The forces which is holding different atoms together in molecules is called bond. The different atoms in a molecule have definite geometric arrangements in space around the central atom. This definite relative arrangement of the bonded atoms in a molecule is known as the geometry or shape of the molecules.
The shape of a covalently bonded molecule depends upon the orientation of the atoms in molecules. Due to repulsive interactions between electron pairs constituting bonds, the bonded atoms adopt that particular arrangement in space around the central atom, which keeps them on the average as far apart as possible. This state will have minimum repulsive interactions and thus corresponds to a state of minimum energy and maximum stability. Since there can be only one definite orientation of electron pairs around the central atom corresponding to state of minimum repulsion (or energy), a molecule can have only one fixed shape or definite geometry.
VSEPR Theory:-
In 1940, Sidgwick and Powell proposed a simple theory based on the repulsive interactions of the electron pairs in the valence shell of the atoms. They suggested that the shapes of molecules can be determined by the number of electron pairs (bonding as well as non-bonding) in the valence shell of the central atom. This suggestion is the basis of valence shell electron pair repulsion (VSEPR) theory. The basic idea of this theory is that bonded atoms in a molecule adopt that particular arrangement in space around the central atom which keeps them on the average as far apart as possible.
Postulates of VSEPR Theory:-
The main postulates of VSEPR theory are:
1) The geometry of a molecule depends upon the number of valence shell electron pairs (whether bonded or not) around the central atom.
2) Electron pairs tend to repel one another because their electron clouds are negatively charged. As a result, the electron pairs try to stay as far apart as possible to acquire a state of minimum energy or maximum stability.
3) Repulsion between the lone pair and lone pair of electrons is different than that between two bond pairs or one lone pair and one bond pair. The repulsive interactions decrease in the order:
Lone pair— Lone pair > Lone pair— Bond pair > Bond pair — Bond pair
The presence of lone pairs in addition to bond pairs will result in certain distortions in the regular geometry of molecules.
4) Repulsive forces decrease sharply with increasing angle between the electron pairs. They are strong at 900, weaker at 120° and weakest at 1800.
Shapes of molecules on the basis of VSEPR Theory:
According to this theory the geometry of a molecule depends on the number of bonding and non-bonding electron pairs around the central atom which arrange themselves in such a way that there is a minimum repulsion between them so that the molecule has minimum energy (i.e. maximum stability). Since there can only be one orientation of orbitals corresponding to minimum energy, the molecule has definite shape.
Rule-1:
If the central atom of a molecule is surrounded by only bonding electron pairs and not by non-bonding electron pairs, called lone pairs, the geometry of the molecule will be regular i.e. linear (BeF2), triangular planar (BF3), tetrahedral (CH4), trigonal bipyramidal (Pcl5), octahedral (SF6) for 2, 3, 4, 5 and 6 bonding electron pairs. For example,
i) Molecules with two bond pairs around the central atom have linear geometry.
ii) Molecules with three bond pairs around the central atom have triangular planar geometry
iii) Molecules with four bond pairs around the central atom have tetrahedral geometry.
iv) Molecules with five bond pairs around the central atom have trigonal bipyramidal geometry.
v) Molecules with six bond pairs around the central atom have an octahedral geometry.
vi) Molecules with seven bond pairs around the central atom have pentagonal bipyramidal geometry.
Rule-2:
When the central atom in a molecule is surrounded by both bonding as well as non-bonding pairs, the molecules do not have a regular shape.
i) Molecules with three bond pairs and one lone pairs around the central atom, has a triangular pyramidal geometry. E.g. NH3, Pcl3, H3O+-ion, etc.
ii) Molecules with two bond pairs and two lone pairs around the central atom have bent (inverted V-shaped) geometry.
iii) Molecules with four bond pairs and two lone pairs around the central atom have square planar geometry. e.g. XeF4, [Ni (CN)4]2-, etc.
