Question

Which one of the following species does not exist under normal conditions?

• A. $Be_2^+$
• B. $Be_2$
• C. $B_2$
• D. $Li_2$

Answer 1

Answer: (B)

Key Idea Molecules with zero bond order, do not exist.
According to molecular orbital theory,
(a) $Be _{2}^{+}(4+4-1=7)$
$=\sigma 1 s^{2}, \overset{*}{\sigma} 1 s^{2}, \sigma 2 s^{2}, \overset{*}{\sigma} 2 s^{1}$
Bond order $( BO )=\frac{4-3}{2}=0.5$
(b) $Be _{2}(4+4=8)=\sigma 1 s^{2}, \overset{*}{\sigma} 1 s^{2}, \sigma 2 s^{2}, \overset{*}{\sigma} 2 s^{2}$
$BO =\frac{4-4}{2}=0$
(c) $B _{2}(5+5=10)$
$=\sigma 1 s^{2}, \overset{*}{\sigma} 1 s^{2}, \sigma 2 s^{2}, \overset{*}{\sigma} 2 s^{2}, \pi 2 p_{x}^{1} \approx \pi 2 p_{y}^{1}$
Bond order $( BO )=\frac{6-4}{2}=1$
(d) $Li _{2}(3+3=6)=\sigma 1 s^{2}, \overset{*}{\sigma} 1 s^{2}, \sigma 2 s^{2}$
$BO =\frac{4-2}{2}=1$
Thus, $Be _{2}$ does not exist under normal conditions.