with the tetrahedral HCH angles of . ° . Clearly, it follows that simple atomic orbital overlap does not account for the directional characteristics of bonds in CH . Using similar procedure and arguments, it can be seen that in the case of NH and H O molecules, the HNH and HOH angles should be ° .
This is in disagreement with the actual bond angles of ° and . ° in the NH and H O molecules respectively. . .
Types of Overlapping and Nature of Covalent Bonds The covalent bond may be classified into two types depending upon the types of overlapping: (i) Sigma( σ ) bond, and (ii) pi( π ) bond (i) Sigma( σ ) bond : This type of covalent bond is formed by the end to end (head- on) overlap of bonding orbitals along the internuclear axis. This is called as head on overlap or axial overlap. This can be formed by any one of the following types of combinations of atomic orbitals. s-s overlapping : In this case, there is overlap of two half filled s -orbitals along the internuclear axis as shown below : s-p overlapping: This type of overlap occurs between half filled s -orbitals of one atom and half filled p -orbitals of another atom.
p–p overlapping : This type of overlap takes place between half filled p -orbitals of the two approaching atoms. (ii) pi( ) bond : In the formation of π bond the atomic orbitals overlap in such a way that their axes remain parallel to each other and perpendicular to the internuclear axis. The orbitals formed due to sidewise overlapping consists of two saucer type charged clouds above and below the plane of the participating atoms. .
. Strength of Sigma and pi Bonds Basically the strength of a bond depends upon the extent of overlapping. In case of sigma bond, the overlapping of orbitals takes place to a larger extent. Hence, it is stronger as compared to the pi bond where the extent of overlapping occurs to a smaller