Question

MO configuration of He is

• A. $(\sigma 1 s)^{2},(\overset{*}{\sigma} 1 s)^{2},(\sigma2 s)^{1}$
• B. $(\sigma 1 s)^{2},(\overset{*}{\sigma} 1 s)^{2},(\overset{*}{\sigma} 2 s)^{1}$
• C. $(\sigma 1 s)^{2},(\overset{*}{\sigma} 1 s)^{2},(\sigma 2 s)^{2}$
• D. $(\sigma 1 s)^{2},(\overset{*}{\sigma} 1 s)^{2},(\sigma 2 s)^{1}$

Answer 1

Answer: (A)

MO configuration of $He _{2}^{-}$
This molecule contains five electrons, two from one helium atom, and three from the other ($He^{-}$ ion). The molecular orbital configuration of the molecule is as
$He _{2}^{-} (\sigma 1 s)^{2}\left(\overset{*}{\sigma} 1 s\right)^{2},(\sigma 2 s)^{1}$
Bond order of $He _{2}^{-}=\frac{N_{b}-N_{a}}{2}=\frac{3-2}{2}=\frac{1}{2}$
and molecule is expected to be paramagnetic due to the presence of unpaired electron.