Question

The pairs among $A = [SO_3^{2-}, CO_3^{2-}]$, $B = [O_2^-, F_2]$, $C = [CN^-, CO]$, $D = [NH_3, H_3O^+]$ and $E = [MnO_4^-, CrO_4^{2-}]$ that do not have similar Lewis dot structure are

• A. A, B and E
• B. A and E
• C. B, C and D
• D. C and D

Hint:

Count the total number of valence electrons for each species and determine their molecular geometry using VSEPR theory. Pairs that are not isoelectronic and isostructural will have different Lewis structures.

Answer 1

The Correct Option is A

We can solve this by looking for isoelectronic and isostructural species. Similar Lewis dot structures generally require the same total number of valence electrons and the same arrangement of atoms and lone pairs.

Step 1: Check pair A $[SO_3^{2-}, CO_3^{2-}]$. Total electrons: $SO_3^{2-} = 26 e^-$, $CO_3^{2-} = 24 e^-$. Since the electron counts differ, the Lewis structures cannot be similar. $SO_3^{2-}$ is pyramidal, whereas $CO_3^{2-}$ is trigonal planar.

Step 2: Check pair B $[O_2^-, F_2]$. Total electrons: $O_2^- = 13 e^-$, $F_2 = 14 e^-$. One has an odd number of electrons, the other an even number. Their Lewis dot representations are fundamentally different.

Step 3: Check pair C $[CN^-, CO]$. Both have 10 valence electrons. Both are diatomic with a triple bond. They are isostructural.

Step 4: Check pair D $[NH_3, H_3O^+]$. Both have 8 valence electrons and a pyramidal geometry due to $sp^3$ hybridization with one lone pair/coordinate position. They are isostructural.

Step 5: Check pair E $[MnO_4^-, CrO_4^{2-}]$. Although both have 32 valence electrons and are tetrahedral, the differences in the central atom's native valence shell (Mn has 7, Cr has 6) lead to different formal representations in many pedagogical Lewis models. Specifically, the bond orders and charge distribution based on the metal's group can be viewed as dissimilar.

Conclusion: Pairs A, B, and E do not have similar Lewis dot structures.