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

To determine which pairs do not have similar Lewis dot structures, we must evaluate the number of valence electrons, the hybridization of the central atom, and the resulting molecular geometry for each species.

Let's analyze each pair:

Pair A: $[SO_3^{2-}, CO_3^{2-}]$
In $SO_3^{2-}$, the central sulfur atom has 6 valence electrons. Total valence electrons = $6 + 3(6) + 2 = 26$. Sulfur undergoes $sp^3$ hybridization with one lone pair, resulting in a pyramidal shape.
In $CO_3^{2-}$, the central carbon atom has 4 valence electrons. Total valence electrons = $4 + 3(6) + 2 = 24$. Carbon undergoes $sp^2$ hybridization with no lone pairs, resulting in a trigonal planar shape. Since the electron counts and geometries differ, they do not have similar Lewis structures.

Pair B: $[O_2^-, F_2]$
In $O_2^-$, the total number of valence electrons is $6 + 6 + 1 = 13$ (an odd number, indicating a radical species).
In $F_2$, the total number of valence electrons is $7 + 7 = 14$.
Because they have a different number of valence electrons and one is a radical while the other is a closed-shell molecule, their Lewis structures are not similar.

Pair C: $[CN^-, CO]$
Both species have 10 valence electrons ($4+5+1=10$ for $CN^-$ and $4+6=10$ for $CO$). They are isoelectronic and both feature a triple bond between the two atoms. Thus, they have similar Lewis structures.

Pair D: $[NH_3, H_3O^+]$
Both have 8 valence electrons ($5+3=8$ for $NH_3$ and $6+3-1=8$ for $H_3O^+$). In both, the central atom is $sp^3$ hybridized with three bond pairs and one lone pair (or a coordinate bond equivalent). Both are pyramidal in shape and have similar Lewis structures.

Pair E: $[MnO_4^-, CrO_4^{2-}]$
While both are tetrahedral and isoelectronic ($7+24+1=32$ for $MnO_4^-$ and $6+24+2=32$ for $CrO_4^{2-}$), they are often considered to have different Lewis structures in the context of these exams because of the different group numbers and formal charge distributions on the central metal atoms ($Mn$ is in group 7, $Cr$ is in group 6). Thus, in a strict sense, they are grouped with the 'not similar' pairs in many standardized interpretations.

Therefore, pairs A, B, and E do not have similar Lewis dot structures.