Question

Which of the following is not a Green House Gas?

• A. Nitrogen
• B. Methane
• C. Nitrous oxide
• D. Carbon dioxide

Hint:

To be a greenhouse gas, a molecule must have three or more atoms (like $H_2O$, $CO_2$, $CH_4$) or be asymmetrical. Symmetrical diatomic molecules that dominate our atmosphere (like $N_2$ and $O_2$) cannot trap heat.

Answer 1

The Correct Option is A

Step 1: Understanding the Greenhouse Effect:
The greenhouse effect is a natural process where specific atmospheric gases absorb infrared thermal radiation emitted from the Earth's surface and re-radiate it in all directions, trapping heat in the atmosphere.

Step 2: Molecular Criteria for Greenhouse Gases:
To absorb infrared radiation, a gas molecule must be able to undergo a change in its net dipole moment when its chemical bonds vibrate.

• Homonuclear Diatomic Molecules: Gases like Nitrogen ($N_2$) and Oxygen ($O_2$) consist of two identical atoms. Since they are completely symmetrical, their vibration does not create a transient electrical dipole moment, making them transparent to infrared light.
• Heteronuclear and Polyatomic Molecules: Gases like Carbon Dioxide ($CO_2$), Methane ($CH_4$), and Nitrous Oxide ($N_2O$) have asymmetrical vibrational modes that easily change their dipole moments, allowing them to absorb thermal radiation.

Step 3: Conclusion:
Nitrogen makes up approximately $78\%$ of the atmosphere, but its homonuclear diatomic molecular structure prevents it from acting as a greenhouse gas. Thus, option (A) is correct.