Question
Use molecular orbital theory to explain why the $Be _2$ molecule does not exist.

Answer

The atomic number of Be is 4 . It means in $Be _2$ molecule 8 electrons are filled. Hence electronic configuration of $Be _2$ molecule is :
$(\sigma 1 s)^2\left(\sigma 1 s^*\right)^2(\sigma 2 s)^2\left(\sigma^* 2 s\right)^2$ or $KK (\sigma 2 s)^2\left(\sigma^* 2 s\right)^2$
Bond order $=\frac{1}{2}[ Nb - Na ]$
$=\frac{1}{2}[4-4]=0$ (zero)
The zero value of bond order represents that $Be _2$ molecule do not exist.

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