Engineering Chemistry S Datta

30 ENGINEERING CHEMISTRY
Table 2.4
B.O.
Significance
0 molecule is unstable
and does not exist.
–ve
+ve
unstable and does not exist.
*stable and exists.
*Greater the value of B.O. greater is the stability of the molecule.
• Rules for filling up of molecular orbitals with electrons:
(i) The molecular orbitals are filled in the increasing order of their energies. Thus molecular
orbital posses3ing the lowest energy will be filled first and that possessing
the highest energy will be filled up in the last.
(ii) The maximum number of electrons that can fill a molecular orbital is two.
(iii) According to Hund’s rule of maximum multiplicity, if there are two molecular orbitals
having the same energy (i.e., degenerate molecular orbitals), the electrons will first
fill them singly and when both of them have been filled singly, pairing of electrons
will occur. Thus π y
2p and π z
2p bonding molecular orbitals which are degenerate are
first singly filled and pairing takes place only when more electrons are to be
accommodated. In a similar fashion π y
*2p and π z
*2p antibonding degenerate molecular
orbitals are filled just like degenerate bonding orbitals.
* 2px
* (sp)2
*
*
2p z
2p y
2p x
2s
2p z
2p z
2p z
2p
2p y 2p
y
y
2p 2p
2p x
x
x
(sp)2
* (sp)1
2s
2s
2s
2s
2px
* 2s
(sp)2
Fig. 2.26 Molecular orbital diagram from Fig. 2.27 Molecular orbital diagram
2s and 2p atomic orbitals. allowing s-p interaction.
• Energy level diagram for N 2
molecule:
The electronic configuration of N-atom is 1s 2 2s 2 2p 3 . In N 2
molecule there will be all
total 14 electrons of which 4 will be in the K shells which is denoted by KK.