Question

According to molecular orbital theory, the species among the following that does not exist is :

• A. O$_2^{2-}$
• B. He$_2^-$
• C. Be$_2$
• D. He$_2^+$

Hint:

Species with B.O. = 0 (like He$_2$, Be$_2$, Ne$_2$) cannot exist because the stability gained from bonding electrons is exactly cancelled by the instability of antibonding electrons.

Answer 1

The Correct Option is C

Step 1: According to MOT, a species does not exist if its Bond Order (B.O.) is zero.
Step 2: For Be$_2$ ($Z=4$, total electrons = 8): Electronic configuration: $\sigma1s^2, \sigma^*1s^2, \sigma2s^2, \sigma^*2s^2$.
Step 3: $\text{B.O.} = \frac{1}{2}(N_b - N_a) = \frac{1}{2}(4 - 4) = 0$.
Step 4: Since the bond order is zero, Be$_2$ is unstable and does not exist under normal conditions.