Question

Choose CORRECT order of bond length

• A. S - S bond length : $S_2O_4^{2-} > S_2O_6^{2-}$
• B. O - O bond length : $H_2O_2 > O_2F_2$
• C. N - N bond length : $N_2O_3 > N_2O_4$
• D. All are correct

Answer 1

Answer: (A), (B), (C), (D)

All are correct

Answer 2

To determine the correct order of bond lengths, we will analyze each option based on known experimental bond lengths and relevant chemical principles.

A) S - S bond length: $S_2O_4^{2-} > S_2O_6^{2-}$

• $S_2O_4^{2-}$ (Dithionite ion): The structure is $^{-O}-S(=O)-S(=O)-O^{-}$. Each sulfur atom is in a +3 oxidation state and is bonded to one other sulfur atom and two oxygen atoms. Each sulfur atom has one lone pair of electrons. The S-S bond length is approximately 2.39 Å. The presence of lone pairs on the sulfur atoms contributes to greater electron-electron repulsion, which tends to lengthen the bond.
• $S_2O_6^{2-}$ (Dithionate ion): The structure is $O_2S(=O)-S(=O)O_2^{2-}$. Each sulfur atom is in a +5 oxidation state and is bonded to one other sulfur atom and three oxygen atoms. There are no lone pairs on the sulfur atoms. The S-S bond length is approximately 2.15 Å. The higher oxidation state and lack of lone pairs on sulfur lead to a shorter S-S bond compared to $S_2O_4^{2-}$.

Since 2.39 Å > 2.15 Å, the order $S_2O_4^{2-} > S_2O_6^{2-}$ is CORRECT.

B) O - O bond length: $H_2O_2 > O_2F_2$

• $H_2O_2$ (Hydrogen Peroxide): The structure is H-O-O-H. The O-O bond is a single bond. The O-O bond length is approximately 1.48 Å.
• $O_2F_2$ (Dioxygen Difluoride): The structure is F-O-O-F. The O-O bond is a single bond. Fluorine is highly electronegative and pulls electron density from the oxygen atoms. This makes the oxygen atoms more electron-deficient, reducing the lone pair-lone pair repulsion between them. This effect, along with a possible increase in effective nuclear charge on oxygen, leads to an unusually short O-O bond length of approximately 1.22 Å. This is even shorter than the O=O double bond in $O_2$ (1.21 Å).

Since 1.48 Å > 1.22 Å, the order $H_2O_2 > O_2F_2$ is CORRECT.

C) N - N bond length: $N_2O_3 > N_2O_4$

• $N_2O_3$ (Dinitrogen Trioxide): The structure is O=N-NO2. The N-N bond is a single bond. The N-N bond length is approximately 1.86 Å.
• $N_2O_4$ (Dinitrogen Tetroxide): The structure is $O_2N-NO_2$. The N-N bond is a single bond. This bond is known to be unusually long and weak due to steric repulsion between the oxygen atoms and electronic repulsion between the positively charged nitrogen atoms. The N-N bond length is approximately 1.75 Å.

Since 1.86 Å > 1.75 Å, the order $N_2O_3 > N_2O_4$ is CORRECT.

Conclusion: All three statements (A), (B), and (C) are correct based on experimental bond length data. Therefore, option (D) is the correct choice.