Question

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

• A. $B{e}_2^{+}$
• B. $B{e}_2$
• C. $B_2$
• D. $L{i}_2$

Answer 1

Answer: (B)

Key Idea Molecules with zero bond order, do not exist.
According to molecular orbital theory,
(a) $B{e}_2^{+}(4+4-1=7)$
$=\sigma 1s^2,\stackrel{\ast }{\sigma }1s^2,\sigma 2s^2,\stackrel{\ast }{\sigma }2s^1$
Bond order $(BO)=\frac{4-3}{2}=0.5$
(b) $B{e}_2(4+4=8)=\sigma 1s^2,\stackrel{\ast }{\sigma }1s^2,\sigma 2s^2,\stackrel{\ast }{\sigma }2s^2$
$BO=\frac{4-4}{2}=0$
(c) $B_2(5+5=10)$
$=\sigma 1s^2,\stackrel{\ast }{\sigma }1s^2,\sigma 2s^2,\stackrel{\ast }{\sigma }2s^2,\pi 2p_x^1\approx \pi 2p_y^1$
Bond order $(BO)=\frac{6-4}{2}=1$
(d) $L{i}_2(3+3=6)=\sigma 1s^2,\stackrel{\ast }{\sigma }1s^2,\sigma 2s^2$
$BO=\frac{4-2}{2}=1$
Thus, $B{e}_2$ does not exist under normal conditions.