Chapter 3: – Chemical Bonding
B.sc 1st year Book
(Page 11)
Sidgwick-Powell theory
Sidgwick and Powell reviewed the known structure of different molecules and suggested that if the molecules or ions have only single bonds then their approximate shape can be predicted from the number of electron pairs (e.p.’s) present in the valence shell of the central atom. The outer shell (or valency shell) of the center may contain either two or more bond pairs (b.p.’s) or bond pairs as well as unshared pairs of electrons called lone pairs (L.p.’s). These electron pairs (e.p.’s) are taken as equivalent because they occupy some space and also they repel each other. The repulsion is carried out in such a way that lone pairs(l.p.’s) and bond pairs (b.p.’s) are oriented in space as far apart as possible.
According to Sidgwick-Powell’s concept
1- If in a molecule there are two bond pairs in the valence shell of the central atom then the orbitals containing them are oriented at 180∘ to each other and the molecule will be linear θ.g. BeCl2.
2- If the central atom of a molecule contains three bond pairs (b.p.’s) in its valence shell then its three bond orbitals are at an angle of 120∘ to each other and its molecule will be triangular planar e.g. BCl3, BF3, etc.
3- If the central atom of a molecule is surrounded by only four bond pairs (b.p.’s) then its four bond orbitals are at an angle of 109∘28∘ to each other and its molecule will be tetrahedral e.g. CH4, CCl4, CH3Cl4CH2Cl2, CHCl3, and SiCl4, etc. Similarly
4- If the central atom of a molecule ts surrounded by only five bond pairs (b.p.’s) then its five bond orbitals are at an angle of 90∘ and 120∘ to each other and its molecule will be trigonal bi-pyramidal, e.g. PCl5.
5- If the central atom of a molecule is surrounded by only six bond pairs (b.p.’s) then its six bond orbitals are at an angle of 90∘ to each other and its molecule will be octahedral e.g. SFG
2- If the central atom of a molecule contains three bond pairs (b.p.’s) in its valence shell then its three bond orbitals are at an angle of 120∘ to each other and its molecule will be triangular planar e.g. BCl3, BF3, etc.
3- If the central atom of a molecule is surrounded by only four bond pairs (b.p.’s) then its four bond orbitals are at an angle of 109∘28∘ to each other and its molecule will be tetrahedral e.g. CH4, CCl4, CH3Cl4CH2Cl2, CHCl3, and SiCl4, etc. Similarly
4- If the central atom of a molecule ts surrounded by only five bond pairs (b.p.’s) then its five bond orbitals are at an angle of 90∘ and 120∘ to each other and its molecule will be trigonal bi-pyramidal, e.g. PCl5.
5- If the central atom of a molecule is surrounded by only six bond pairs (b.p.’s) then its six bond orbitals are at an angle of 90∘ to each other and its molecule will be octahedral e.g. SFG
6- If the central atom of a molecule is surrounded by only seven bond pairs (b.p.’s) then its seven bond orbitals are at an angle of 72∘ and 90∘ to each other and its molecule will be pentagonal bipyramidal e.g. IF7. as shown in table 3.5.
Note. The electron pairs which involve in bonding between the atoms ate called bond pairs’ (bp’s) and those electron pairs which do not involve in bonding are called “non-bonding. electron pairs’ or lone pairs (lp’s).