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

The Correct Option is B

Molecules with zero bond order, do not exist. According to molecular orbital theory,
(a) $Be_2^ + (4+4-1=7)$
$ \, \, \, \, \, \, \, \, \, \, \, \, \, = \sigma 1s^2, _{\sigma}^{*} 1s^2, \sigma 2s^2,_{\sigma}^{*} 2s^1 $
Bond order (BO) = $ \frac{4-3}{2}=0.5$
(b) $Be_2(4+5=8) = \sigma 1s^2 , _{\sigma}^{*} 1s^2, _{\sigma}^{*} 1s^2, \sigma 2s^2, _{\sigma}^{*} 2s^2$
$ \, \, \, \, \, \, \, \, BO = \frac{ 4-4}{2}= 0 $
(c) $B_2 (5+5=10)$
$ \, \, \, \, \, = \sigma 1s^2 , _{\sigma}^{*} 1s^2, \sigma 2s^2 ,_{\sigma}^{*} 2s^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) = \sigma1s^2,_{\sigma}^{*} 1s^2,\sigma2s^2$
$ \, \, \, \, \, \, \, BO = \frac{4-2}{2} =1$
Thus, $Be_2 $ does not exist under normal conditions.