Question

Which of the following orders is not correct for the property mentioned against them?

• A. $\text{H}_{2}\text{O} > \text{HF} > \text{NH}_{3} > \text{H}_{2}\text{S}$ (boiling point)
• B. $\text{H}_{2}\text{O} > \text{NH}_{3} > \text{NF}_{3} > \text{CF}_{4}$ (dipole moment)
• C. $\text{C}-\text{C} > \text{N}-\text{O} > \text{C}-\text{O} > \text{C}-\text{H}$ (bond length)
• D. $\text{O}_{2}^{+} > \text{O}_{2} > \text{O}_{2}^{-} > \text{O}_{2}^{2-}$ (bond order)

Hint:

Smaller atoms form tighter, shorter covalent links. Because carbon atoms are larger than nitrogen atoms, a $\text{C}-\text{O}$ link must be longer than an $\text{N}-\text{O}$ link.

Answer 1

The Correct Option is C

Step 1: Read the task.
Each option lists an order for some property. We must find the one order that is wrong.

Step 2: Check the boiling point order.
Water has strong hydrogen bonding, so it boils highest, then HF, then ammonia, while $H_2S$ has no hydrogen bonding and boils lowest. So $H_2O > HF > NH_3 > H_2S$ is correct.

Step 3: Check the dipole moment order.
In water the two lone pairs add to the polarity. In $NF_3$ the lone pair pulls against the bond polarity, lowering the dipole. So $H_2O > NH_3 > NF_3 > CF_4$ is correct, and $CF_4$ is non polar.

Step 4: Check the bond order list.
Using molecular orbital ideas, $O_2^+$ has bond order 2.5, $O_2$ is 2.0, $O_2^-$ is 1.5 and $O_2^{2-}$ is 1.0. So $O_2^+ > O_2 > O_2^- > O_2^{2-}$ is correct.

Step 5: Check the bond length list.
Bond length depends on atom size. Carbon is bigger than nitrogen, so a $C-O$ single bond is longer than an $N-O$ single bond. The option claims $N-O > C-O$, which reverses the true order.

Step 6: Pick the wrong order.
So the bond length order is the incorrect one. \[ \boxed{\text{The bond length order in option (3) is wrong}} \]