and so the bonding influence is stronger and a stable molecule results. In (ii) the antibonding influence is stronger and therefore the molecule is unstable. Bond order Bond order (b.o.) is defined as one half the difference between the number of electrons present in the bonding and the antibonding orbitals i.e., Bond order (b.o.) = ½ (N b –N a ) The rules discussed above regarding the stability of the molecule can be restated in terms of bond order as follows: A positive bond order (i.e., N b > N a ) means a stable molecule while a negative (i.e., N b <N a ) or zero (i.e., N b = N a ) bond order means an unstable molecule. Nature of the bond Integral bond order values of , or correspond to single, double or triple bonds respectively as studied in the classical concept.
Bond-length The bond order between two atoms in a molecule may be taken as an approximate measure of the bond length. The bond length decreases as bond order increases. Magnetic nature If all the molecular orbitals in a molecule are doubly occupied, the substance is diamagnetic (repelled by magnetic field). However if one or more molecular orbitals are singly occupied it is paramagnetic (attracted by magnetic field), e.g., O molecule.
. BONDING IN SOME HOMONUCLEAR DIATOMIC MOLECULES In this section we shall discuss bonding in some homonuclear diatomic molecules. . Hydrogen molecule (H ) : It is formed by the combination of two hydrogen atoms.
Each hydrogen atom has one electron in s orbital. Therefore, in all there are two electrons in hydrogen molecule which are present in σ 1s molecular orbital. So electronic configuration of hydrogen molecule is H : ( σ 1s ) The bond order of H molecule can be calculated as given below: Bond order = N N b a This means that the two hydrogen atoms are bonded together by a single covalent bond. The bond dissociation energy of hydrogen molecule has been found to be