mot(molecular orbital theory) is different for above 14 electron 

Dear student,

Electronic configuration of molecules containing 14 or less electrons :
σ1s , σ*1s , σ2s , σ*2s , π2pxπ2py , σ2pz , π*2pxπ*2py , σ*2pz
Electronic configuration of molecules containing 15 or more electrons :
σ1s , σ*1s , σ2s , σ*2s , σ2pz , π2pxπ2py , π*2pxπ*2py , σ*2pz
  • This difference is dependent on the decreasing value of 's-p bonding interactions' i.e. bond interactions between 2s orbital of one atom and 2p of another atom.
  • Now on moving from left to right in the periodic table i.e. from Li2 to F2, the energy difference between 2s and 2p orbitals increases due to increase in nuclear charge and lower shielding of 2s electrons by 2p electrons.
  • Due to which, increasing s-p interactions on σ2pz becomes less important.
  • Hence, after N2 molecule the energy of σ2pz molecular orbital becomes lower than that of π2px and π2py.
Hope this information will clear your doubts about the topic.

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