Which one of the following gases is not a green ‘house gas?

The correct answer is D) O₂. Oxygen is not a greenhouse gas because it is a homonuclear diatomic molecule. It has no permanent dipole moment, and its stretching vibration does not change the dipole, so it does not absorb the infrared radiation that Earth emits. Its main absorption is in the UV range, which is why it matters for ozone formation in the stratosphere.

• A) CO₂: A linear triatomic molecule whose asymmetric stretch and bending modes absorb infrared. It is the principal long-lived greenhouse gas and is in the Kyoto basket.
• B) NO₂: A polyatomic molecule that absorbs infrared. It is short-lived, and its warming effect is mainly indirect, through tropospheric ozone and nitrate aerosols. So it is not the answer the question wants.
• C) CH₄: Methane is a potent greenhouse gas, roughly 28 times CO₂ per unit mass over a century (AR5 GWP), and is in the Kyoto basket.

The core concept is the infrared-activity condition: a gas must have a changing dipole moment during vibration to be a greenhouse gas. This links directly to Mains GS-III on climate change, the Kyoto Protocol gas basket, and the Paris Agreement.

Trap: Aspirants often mark NO₂ because it is a pollutant, or confuse it with nitrous oxide (N₂O), which is a well-known greenhouse gas. Remember that being a pollutant or being abundant in the atmosphere does not make a gas a greenhouse gas.