The increasing order of energies of various molecular orbitals are

$\text{ I. }\mathrm{\sigma }1\mathrm{s}<{\sigma }^{*}1\mathrm{s}<\mathrm{\sigma }2\mathrm{s}<{\sigma }^{*}2\mathrm{s}<\mathrm{\sigma }2{\mathrm{p}}_{\mathrm{z}}<$$\left(\mathrm{\pi }2{\mathrm{p}}_{\mathrm{x}}=\mathrm{\pi }2{\mathrm{p}}_{\mathrm{y}}\right)<\left({\mathrm{\pi }}^{*}2{\mathrm{p}}_{\mathrm{x}}={\mathrm{\pi }}^{*}2{\mathrm{p}}_{\mathrm{y}}\right)<{\sigma }^{*}2{\mathrm{p}}_{\mathrm{z}}$

$\text{ II. }\mathrm{\sigma }1\mathrm{s}<{\sigma }^{*}1\mathrm{s}<\mathrm{\sigma }2\mathrm{s}<{\sigma }^{*}2\mathrm{s}<\left(\mathrm{\pi }2{\mathrm{p}}_{\mathrm{x}}=\mathrm{\pi }2{\mathrm{p}}_{\mathrm{y}}\right)$$<\mathrm{\sigma }2{\mathrm{p}}_{\mathrm{z}}<\left({\mathrm{\pi }}^{*}2{\mathrm{p}}_{\mathrm{x}}={\mathrm{\pi }}^{*}2{\mathrm{p}}_{\mathrm{y}}\right)<{\mathrm{\sigma }}^{*}2{\mathrm{p}}_{\mathrm{z}}$

Choose the correct option.

detailed solution

Correct option is C

The increasing order of energies of various molecular orbitals for O2 and F2 (no. of e- >14) is given belowσ1s<σ*1s<σ2s<σ*2s<σ2pz<π2px=π2py<π*2px=π*2py<σ*2pzHowever, this sequence of energy levels of molecular orbitals is not correct for the remaining molecules Li2,Be2,   B2,  C2, N2. (no. of e- ≤14) For instance, it has been observed experimentally that for molecules such as B2,C2, N2 etc. the increasing order of energies of various molecular orbitals is σ1s<σ*1s<σ2s<σ*2s<(π2px=π2py<σ2pz <π*2px=π*2py<σ*2pzThe important characteristic feature of this order is that the energy of σ2pzmolecular orbital is higher than that of π2px and π2pymolecular orbitals.