Question

Bond order normally gives an idea of the stability of a molecular species. All the molecules viz. ${\mathrm{H}}_2,{\mathrm{Li}}_2\mathrm{and}{\mathrm{B}}_2$ have the same bond order yet they are not equally stable. Their stability order is:

• A. ${\mathrm{H}}_2>{\mathrm{B}}_2>{\mathrm{Li}}_2$
• B. ${\mathrm{H}}_2>{\mathrm{Li}}_2>{\mathrm{B}}_2$
• C. ${\mathrm{Li}}_2>{\mathrm{B}}_2>{\mathrm{H}}_2$
• D. ${\mathrm{B}}_2>{\mathrm{H}}_2>{\mathrm{Li}}_2$

Answer 1

The correct option is A

${\mathrm{H}}_2>{\mathrm{B}}_2>{\mathrm{Li}}_2$

Explanation for correct option:

(A) ${\mathrm{H}}_2>{\mathrm{B}}_2>{\mathrm{Li}}_2$

• The configuration of the three elements is as follows:
• $$\begin{gathered} {\mathrm{H}}_2=1{\mathrm{S}}^2-\mathrm{no}\mathrm{antibonding}\mathrm{electrons} \\ {\mathrm{Li}}_2=1{\mathrm{S}}^{2}2{\mathrm{S}}^2-2\mathrm{antibonding}\mathrm{electrons} \\ {\mathrm{B}}_2=1{\mathrm{S}}^{2}2{\mathrm{S}}^{2}2{\mathrm{p}}^1-4\mathrm{antibonding}\mathrm{electrons} \end{gathered}$$
• We'll observe that hydrogen has no electrons when it forms an antibonding orbital.
• ${\mathrm{Li}}_2$ and ${\mathrm{B}}_2$ both have two antibonding electrons and four antibonding electrons, respectively.
• The lower the stability of the element, the higher will be the number of the antibonding electron.
• This affects the bond order's stability, thus the order will be this ${\mathrm{H}}_2>{\mathrm{B}}_2>{\mathrm{Li}}_2$ which is the correct option

Explanation for incorrect options:

(B) ${\mathrm{H}}_2>{\mathrm{Li}}_2>{\mathrm{B}}_2$

• This order does not match the antibonding filling of electrons and their stability. So, this is an incorrect option.

(C) ${\mathrm{Li}}_2>{\mathrm{B}}_2>{\mathrm{H}}_2$

• This order does not match the antibonding filling of electrons and their stability. So, this is an incorrect option.

(D) ${\mathrm{Li}}_2>{\mathrm{B}}_2>{\mathrm{H}}_2$

• This order does not match the antibonding filling of electrons and their stability. So, this is an incorrect option.

Hence, the correct option is (A) ${\mathrm{H}}_2>{\mathrm{B}}_2>{\mathrm{Li}}_2$.