(v) The shapes of molecular orbitals depend upon the shapes of combining atomic orbitals. (vi) The bonding molecular orbitals are represented by σ (sigma), π (pi), δ (delta) and the antibonding molecular orbitals are represented by σ∗, π∗, δ*. (vii) The molecular orbitals are filled in the increasing order of their energies, starting with orbital of least energy. (Aufbau principle). (viii) A molecular orbital can accommodate only two electrons and these two electrons must have opposite spins. (Paul’s exclusion principle). (ix) While filling molecular orbitals of equal energy, pairing of electrons does not take place until all such molecular orbitals are singly filled with electrons having parallel spins. (Hund’s rule). 1.5.1 Energy level diagram for molecular orbitals In case of homonuclear diatomic molecules, combination of two 1s atomic orbitals of participating atoms give rise to two new molecular orbitals designated as σ1s and σ*1s. In the same manner the 2s and three 2p-orbitals of each atom i.e., eight atomic orbitals can give rise to eight new molecular orbitals viz., ó 2s , ó*2s , ð 2p x , ð*2p x , ð 2p y , ð*2p y , ó 2p z , ó *2p z . Atomic Structure and Chemical Bonding Energy levels of these molecular orbitals have been determined experimentally by spectroscopic studies.The order of increasing energy in case of the diatomic homonuclear molecules of first and second period of the periodic table is as given below: ó1s < ó1s* < ó 2s < ó*2s < ð 2p x = ð 2p y < ó 2p z < ð*2p x = ð*2p y < ó *2p z This order of energies of various molecular orbitals is valid for molecules or ions like, H2, H2+, He2+, He2 (hypothetical), Li2, Be2 (hypothetical), B2, C2 and N2 molecules. This energy diagram for the molecular orbitals is shown in Fig.1.7a. However, experimental evidence for oxygen and heavier diatomic molecules have shown that above sequence of energy levels of MOs is not correct. In case of these elements, the order of energy levels of ó 2p z , ð2px and ð2py is reversed i.e., ó 2p z has lesser energy than ð 2p x or ð 2p y . Thus, the order of increasing energy of MOs for these molecules is as follows. ó 1s < ó 1s* < ó 2s < ó *2s < ó 2p z < ð 2p x = ð 2p y < ð *2p x = ð *2p y < ó *2p z 22

Std12-Chem-EM-1