Q.Using simple Lewis (electron-dot) structures, determine the bond order of the nitrogen–nitrogen bond in $\text{N}_2$ and the oxygen–oxygen bond in $\text{O}_2$. Which of the two should have the shorter, stronger bond, and why?

[!TLDR] The Lewis structure of $\text{N}_2$ is $:\text{N}\equiv\text{N}:$ (bond order 3); that of $\text{O}_2$ is $:\text{O}=\text{O}:$ (bond order 2). [!ANSWER] $\text{N}_2$ has bond order $3$ and $\text{O}_2$ has bond order $2$, so $\text{N}_2$'s triple bond is shorter and stronger than $\text{O}_2$'s double bond.

Nitrogen has 5 valence electrons, so $\text{N}_2$ has $10$ valence electrons total. Distributing these to give each nitrogen an octet requires three shared pairs between the two atoms (a triple bond) plus one lone pair on each nitrogen: $:\text{N}\equiv\text{N}:$, giving bond order $3$. Oxygen has 6 valence electrons, so $\text{O}_2$ has $12$ valence electrons total. Giving each oxygen an octet in the simple Lewis picture requires two shared pairs (a double bond) plus two lone pairs on each oxygen: $:\!\!\overset{\displaystyle \cdot\cdot}{\text{O}}\!=\!\overset{\displaystyle \cdot\cdot}{\text{O}}\!\!:$, giving bond order $2$. Since bond order and bond strength/length are directly correlated (as established in the bond-parameters section of this chapter), the triple-bonded $\text{N}_2$ bond is shorter and its bond dissociation energy is considerably larger than that of the double-bonded $\text{O}_2$ bond — consistent with the experimental values of about $945\ \text{kJ mol}^{-1}$ for $\text{N}_2$ versus about $498\ \text{kJ mol}^{-1}$ for $\text{O}_2$. [!ANSWER] $\text{N}_2$ (bond order 3, triple bond) has the shorter, stronger bond compared to $\text{O}_2$ (bond order 2, double bond).

Count total valence electrons for each molecule, distribute them to satisfy the octet rule on both atoms, read off the bond order from the resulting Lewis structure, then apply the bond-order/bond-strength correlation.

Forgetting to include the lone pairs when counting total electrons; assuming bond order alone (without discussing the correlation) answers 'which is stronger'.