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:
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It has a permanent dipole moment
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It can undergo rotational transitions
Non-polar molecules do NOT absorb microwave radiation.
Option-wise Explanation
Option (A): CO₂ (Carbon Dioxide)
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Linear molecule: O=C=O
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Symmetrical structure
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Dipole moments cancel each other
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Net dipole moment = 0
❌ Does NOT absorb microwave radiation
Correct Answer
Option (B): H₂O (Water)
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Bent molecular geometry
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Highly polar molecule
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Strong permanent dipole moment
✔ Absorbs microwave radiation
✔ Basis of microwave heating
Not the correct option
Option (C): CO (Carbon Monoxide)
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Linear but heteronuclear
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Small but non-zero dipole moment
✔ Absorbs microwave radiation
Not the correct option
Option (D): NO (Nitric Oxide)
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Diatomic heteronuclear molecule
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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
| Molecule | Dipole Moment | Microwave Absorption |
|---|---|---|
| CO₂ | 0 | ❌ No |
| H₂O | High | ✔ Yes |
| CO | Small | ✔ Yes |
| NO | Present | ✔ Yes |
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.
