Question:medium

In which of the following, the species are arranged in the decreasing order of their bond dissociation enthalpies?

Show Hint

Bond order is directly proportional to bond dissociation enthalpy in MO theory.
Updated On: Jul 18, 2026
  • O\(_2^+\) > O\(_2\) > O\(_2^-\) > O\(_2^{2-}\)
  • O\(_2\) > O\(_2^-\) > O\(_2^+\) > O\(_2^{2-}\)
  • O\(_2^+\) > O\(_2\) > O\(_2^{2-}\) > O\(_2^-\)
  • O\(_2\) > O\(_2^+\) > O\(_2^-\) > O\(_2^{2-}\)
Show Solution

The Correct Option is A

Solution and Explanation

Step 1: Build the valence molecular orbital filling for neutral $O_2$.
Oxygen has 12 valence electrons per molecule (6 from each atom), and since $O_2$ has more electrons than $N_2$, the filling order is:
\[ \sigma 2s^2\, \sigma^{*}2s^2\, \sigma 2p_z^2\, \pi 2p_x^2\,\pi 2p_y^2\, \pi^{*}2p_x^1\,\pi^{*}2p_y^1 \]

Step 2: Separate bonding electrons from antibonding electrons.
Bonding: $\sigma 2s^2 + \sigma 2p_z^2 + \pi 2p_x^2 + \pi 2p_y^2 = 8$.
Antibonding: $\sigma^{*}2s^2 + \pi^{*}2p_x^1 + \pi^{*}2p_y^1 = 4$.
\[ \text{Bond order} = \frac{8-4}{2} = 2 \]

Step 3: Remove one electron for $O_2^{+}$.
Removing an electron takes it from the highest occupied, weakest-bound orbital, which is $\pi^{*}$. Antibonding count drops to $3$:
\[ \text{Bond order}(O_2^{+}) = \frac{8-3}{2} = 2.5 \]

Step 4: Add one electron for $O_2^{-}$.
The extra electron also enters $\pi^{*}$, raising antibonding count to $5$:
\[ \text{Bond order}(O_2^{-}) = \frac{8-5}{2} = 1.5 \]

Step 5: Add a second electron for $O_2^{2-}$.
Both $\pi^{*}$ orbitals are now completely filled, giving antibonding count $6$:
\[ \text{Bond order}(O_2^{2-}) = \frac{8-6}{2} = 1 \]

Step 6: Rank by bond order, since a higher bond order always means a stronger, harder-to-break bond.
\[ O_2^{+}(2.5) \gt O_2(2) \gt O_2^{-}(1.5) \gt O_2^{2-}(1) \]

Final Answer:
\[ \boxed{O_2^{+} \gt O_2 \gt O_2^{-} \gt O_2^{2-}} \]
Was this answer helpful?
0