Question

The molecules that possess $\sigma$ (plane of symmetry) as the ONLY element of symmetry are

• A. II and IV
• B. I and IV
• C. III and IV
• D. I and III

Hint:

For molecules having only $\sigma$ symmetry, check that the molecule has a mirror plane but no $C_n$ axis, centre of symmetry, or improper rotation axis

Answer 1

The Correct Option is D

Step 1: What is asked.
We need molecules whose only symmetry element is a single mirror plane, with no axis or centre of symmetry.

Step 2: Molecule I.
A flat ring with two different groups lies in one plane, so the molecular plane is its only symmetry element.

Step 3: Molecule III.
Dimethyl sulfoxide is pyramidal at sulfur and has just one plane that passes through the S to O bond.

Step 4: The others.
Molecules II and IV carry extra symmetry, so they are ruled out.

Step 5: Answer.
\[ \boxed{\text{I and III}} \]