Q.12 The molecule that does NOT absorb the microwave radiation is
(A) CO2 (B) H2O
(C) CO (D) NO

Introduction

Microwave spectroscopy is based on the rotational motion of molecules. Not all molecules absorb microwave radiation. For a molecule to absorb microwaves, it must possess a permanent electric dipole moment.

In this article, we analyze the given options and determine which molecule does NOT absorb microwave radiation, a frequently asked question in physical chemistry, JEE, NEET, and GATE exams.

Correct Answer

✅ Option (A): CO₂

Basic Concept: Microwave Absorption

Condition for Microwave Absorption

A molecule absorbs microwave radiation only if:

• It has a permanent dipole moment

• It can undergo rotational transitions

👉 Non-polar molecules do NOT absorb microwave radiation.

Option-wise Explanation

Option (A): CO₂ (Carbon Dioxide)

• Linear molecule: O=C=O

• Symmetrical structure

• Dipole moments cancel each other

• Net dipole moment = 0

❌ Does NOT absorb microwave radiation

✅ Correct Answer

Option (B): H₂O (Water)

• Bent molecular geometry

• Highly polar molecule

• Strong permanent dipole moment

✔ Absorbs microwave radiation
✔ Basis of microwave heating

❌ Not the correct option

Option (C): CO (Carbon Monoxide)

• Linear but heteronuclear

• Small but non-zero dipole moment

✔ Absorbs microwave radiation

❌ Not the correct option

Option (D): NO (Nitric Oxide)

• Diatomic heteronuclear molecule

• Has a permanent dipole moment

✔ Absorbs microwave radiation

❌ Not the correct option

Why CO₂ Does Not Absorb Microwave Radiation

✔ Linear and symmetrical
✔ Zero dipole moment
✔ No rotational microwave transitions

Summary Table

Final Conclusion

Only molecules with a permanent dipole moment can absorb microwave radiation. Since CO₂ is non-polar, it does not absorb microwave radiation, making it the correct answer.

Final Answer

🟢 Correct Option: (A) CO₂