Any gaseous substance in our atmosphere that can absorb energy from infrared radiation (IR) has the potential to cause warming.
In Topic 2, you learned how to determine if a molecule has an evenly distributed electrostatic charge. A molecule will absorb IR radiation if the distribution of charge on the molecule changes during a vibration. This is true for at least some of the vibrations that occur in any molecule that contains at least three atoms. For diatomic molecules, if the two atoms are of different elements, the distribution of charge will change during a vibration. Therefore, all molecular substances, except for homonuclear diatomic molecules, can absorb IR radiation and act as greenhouse gases.
Question: For each molecule below, determine if it could act as a ‘greenhouse gas' (click on the box to check your answer).
Water is a greenhouse gas as the distribution of charge on the molecule can change during a vibration.
Molecular nitrogen is not a greenhouse gas because its charge is evenly distributed and cannot change during a vibration.
Molecular oxygen is made up of two of the same type of atom, so the charge on the molecule is evenly distributed and cannot change during a vibration. Oxygen does not act as a greenhouse gas.
Although CO2 is a linear molecule, the charge distribution on the molecule can change during a vibration. The molecule can undergo bending and asymmetric stretching vibrations which will change the charge distribution. As a result, CO2 can absorb IR radiation and is a greenhouse gas.
Although nitrous oxide is a linear molecule, it is not symetric and can easily have a charge distribution which changes when the bonds in the molecule stretch and bend. As a result it can absorb IR radiation and is a greenhouse gas.