The covalent bonds can be classified into two different categories depending upon the type of overlapping. These are:
(a) Sigma covalent bond
(b) Pi covalent bond.
(a) Sigma ( σ) Bond
This type of covalent bond is formed by the axial overlapping of half-filled atomic orbitals. The atomic orbitals overlap along the inter-nuclear axis and involve end to end or head’ on overlap. The electron cloud formed as a result of axial overlap is cylindrically symmetrical about inter-nuclear axis. The electrons constituting sigma bond are called sigma electrons. There· can be three types of axial overlap among s and p-orbitals as discussed below:
(i) s-s overlap. It involves mutual overlap of half-filled s-orbitals of the atoms approaching to form a bond. The bond formed is called s-s σ bond.
(ii) s-p overlap. It involves mutual overlap of half-filled s-orbital of the one atom with half-filled p-orbital of the other. The bond so formed is called s-p σ bond.
(iii) p-p overlap. It involves mutual overlap of half-filled p-orbitals of the two atoms. The bond so formed is called p-p σ bond.
The s -s, s-p and p-p overlaps have been shown diagrammatically in Fig. 9.1.
Fig. 9.1. s-s, s-p and p-p overlap of atomic orbitals
Strength of three types of sigma bonds. The strength of three types of sigma bonds varies as follows:
p-p > p-s > s-s
It is because of the fact, that p-orbitals allow overlap to greater extent as compared top-s which is larger as compared to s-s overlap.
(b) Pi 17t) Bond
This type of covalent bond is formed by the lateral or sidewise overlap of the atomic orbitals. The orbital overlap takes place in such a way that their axes are parallel to each other but perpendicular to the inter-nuclear axis. The pi bond consists of two charge clouds above and below the plane of the atoms involved in the bond formation. The electrons involved in the n-bond formation are called n-electrons.
Fig. 9.2. Sidewise overlap of p-orbitals.
It may be noted that:
(i) Sigma bond is stronger than pi bond. It is because of the fact that overlapping of atomic orbitals can take place to a greater extent during the formation of sigma bond whereas overlapping of orbitals occurs to a smaller extent during the formation of pi bond.
(ii) Pi bond between the two atoms is formed only in addition to a sigma bond. It is because of the fact that the atoms constituting a single bond prefer to form a strong sigma bond rather than a weak pi bond. Thus, pi bond is always present in molecules having multiple bonds, i.e., double or triple bond. In other words, a single bond cannot be a pi bond.
(iii) The shape of molecule is controlled by the sigma framework (orientations of sigma bonds) around the central atom. Pi bonds are superimposed on sigma bonds hence they simply modify the dimensions of the molecule.
COMPARISON BETWEEN SIGMA AND PI BONDS
The various points of distinction between sigma and pi bonds are given in Table 9.1.
Table 9.1. Comparison of Sigma and Pi Bonds
1. This bond is formed by the axial overlap of atomic orbitals.
2. This bond can be formed by overlap of s-s, s-p or p-p orbitals.
3. The bond is stronger because overlapping can take place to a larger extent.
4. The electron cloud formed by axial overlap is symmetrical about the internuclear axis and consists of single charged cloud.
5. There can be a free rotation of atoms around the σ bond.
6. The bond may be present between the two atoms either alone or along with
7. The shape of molecule is determined by the sigma framework around the central atom.
1. The bond is formed by the sidewise overlap of atomic orbitals.
2. It involves of overlap of p-orbitals only.
3. The bond is weaker because the overlapping occurs to a smaller extent.
4. The electron cloud of pi bond is discontinuous and consists of two charged
clouds above and below the plane of atoms.
5. Free rotation of atoms around 1t bond is not possible because it involves breaking of 1t bond.
6. The bond is always present between the two atoms along with the sigma bond, i.e., it is always superimposed on sigma bond.
7. The 1t bonds do not contribute to the shape,
however, _they modify the dimensions of the molecule
Formation of sigma and pi bonds in F2, HF and O 2 molecules are given in Fig. 9.3
Fig. 9.3. Formation of F 2, HF and 0 2 molecules.