A) \[{{(\sigma 1s)}^{2}},{{(\overset{*}{\mathop{\sigma }}\,1s)}^{2}},{{(\sigma 2s)}^{1}}\]
B) \[{{(\sigma 1s)}^{2}},{{(\overset{*}{\mathop{\sigma }}\,1s)}^{2}},{{(\overset{*}{\mathop{\sigma }}\,2s)}^{1}}\]
C) \[{{(\sigma 1s)}^{2}},{{(\overset{*}{\mathop{\sigma }}\,1s)}^{2}},{{(\sigma 2s)}^{2}}\]
D) \[{{(\sigma 1s)}^{2}},{{(\overset{*}{\mathop{\sigma }}\,1s)}^{2}},{{(\sigma 2s)}^{1}}\]
Correct Answer: A
Solution :
MO configuration of \[He_{2}^{-}\] This molecule contains five electrons, two from one helium atom, and three from the other (\[H{{e}^{-}}\]ion). The molecular orbital configuration of the molecule is as \[He_{2}^{-}\,\,\,\,\,{{(\sigma 1s)}^{2}}{{(\overset{*}{\mathop{\sigma }}\,\,1s)}^{2}},\,{{(\sigma 2s)}^{1}}\] Note - 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.
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