σ ∗ 2p z π ∗ 2p x π ∗ 2p y Bonding MOs σ 2s σ 2p z π 2p x π 2p y The energy levels of these molecular orbitals have been determined experimentally from spectroscopic data for homonuclear diatomic molecules of second row elements of the periodic table. The increasing order of energies of various molecular orbitals for O and F is given below: 1s < ∗ 1s < 2s < ∗ 2s < 2p z < ( π 2p x = π 2p y ) < ( π ∗ 2p x = π ∗ 2p y ) < ∗ 2p z However, this sequence of energy levels of molecular orbitals is not correct for the remaining molecules Li , Be , B , C , N . For instance, it has been observed experimentally that for molecules such as B , C , N , etc. the increasing order of energies of various molecular orbitals is 1s < ∗ 1s < 2s < ∗ 2s < ( π p x = π p y ) < 2p z < ( π ∗ 2p x = π ∗ 2p y ) < ∗ 2p z The important characteristic feature of this order is that the energy of 2p z molecular orbital is higher than that of 2p x and 2p y molecular orbitals .
. . Electronic Configuration and Molecular Behaviour The distribution of electrons among various molecular orbitals is called the electronic configuration of the molecule . From the electronic configuration of the molecule, it is possible to get important information about the molecule as discussed below.
Stability of Molecules: If N b is the number of electrons occupying bonding orbitals and N a the number occupying the antibonding orbitals, then (i) the molecule is stable if N b is greater than N a , and (ii) the molecule is unstable if N b is less than N a . In (i) more bonding orbitals are occupied