Which of the following is not a greenhouse gas?

Concept

A greenhouse gas is any atmospheric gas whose molecules absorb infrared (longwave) radiation emitted by Earth's surface and re-emit it, trapping heat via the greenhouse effect. A gas is infrared-active only when some vibrational mode of its molecule changes the molecule's dipole moment — which needs more than one atom joined by a bond that can stretch or bend.

Application

• Carbon dioxide (CO2) is a linear triatomic molecule; although its symmetric-stretch vibration leaves the dipole moment unchanged, its bending and asymmetric-stretch vibrations do change the dipole moment, so CO2 absorbs and re-emits infrared radiation — making it one of the principal greenhouse gases.

• Nitrous oxide (N2O) is an asymmetric triatomic molecule that is also infrared-active; molecule for molecule it traps far more heat than carbon dioxide, so it too is a potent greenhouse gas.

• Methane (CH4) has C–H bonds whose vibrations change the molecule's dipole moment, making it infrared-active; over a 100-year horizon it traps several times more heat per molecule than carbon dioxide, so it is a significant greenhouse gas as well.

• Helium (He) is a monatomic noble gas — a single atom with no chemical bonds — so it has no vibrational modes at all. With no vibrational motion able to alter a dipole moment, it does not meaningfully absorb or re-emit the infrared radiation that drives the greenhouse effect.

Cross-check

Among the four, only helium's molecule has no vibrational mode capable of interacting with infrared radiation, so it is the one substance that does not meaningfully trap heat in the atmosphere — confirming it is the gas that is not a greenhouse gas.