We have learnt in MOT that atomic orbitals of similar energy and similar symmetry get overlapped to form molecular orbitals. This concept is well accepted when we work on molecules of same element but there is some controversy when we talk about molecules of different elements. Since atomic orbitals belonging to different elements have different energy levels, so in that case we cannot get molecular orbitals by linear combination of atomic orbitals.
Although LCAO can be applied for heteroatomic molecules formed by elements which are closely placed in periodic table. Let’s take an example of NO molecule.
Because the energy levels of MOs are not known exactly, we have to try which order will be well fitted. Let’s take the energy order we have used for heavier atoms like O2.
NO molecule has got 15 electrons, 7 electrons from N and 8 electrons from O atom. Thus we get electronic configuration:
σ1s2, σ*1s2, σ2s2, σ*2s2, σ2px2, {π2py2, π2pz2}, {π*2py1, π*2pz0}
The NO molecule is paramagnetic since it contains an unpaired electron and the bond order will be (10-5)/2=2.5. Here you might get surprised by fractional bond order. It means the bond will be somewhat in between double and triple bond.
The fractional bond order has great significance in the world of molecules. This is the second most important discovery of MOT. This incomplete bond gives extra stability to the multi atomic molecules. In the next post we will explore it in detail.
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