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Key Idea 5
Greenhouse Gases
Concept Question: How can you tell if a gas is an important greenhouse gas?

Greenhouse gases have been mentioned throughout this lesson and are featured in most discussions of climate change, but we have not actually defined greenhouse gases. In this section of the lesson we will explore what characteristics determine if a gas will be a greenhouse gas, and if so, whether it will be an important one.

 Volcano eruption

First of all, what are greenhouse gases? Greenhouse gases are gases found in our troposphere that are capable of absorbing and re-emitting IR radiation. You have already learned about several substances that absorb IR radiation, including CO2, H2O and CFCs. Each of these is an important greenhouse gas.


What do we know?

Since the definition of a greenhouse gas is based on absorption of IR radiation, what characteristics determine if a gas molecule is able to absorb IR radiation? Absorption of IR radiation depends on the distribution of charge when vibrations occur in gas molecules. A gas molecule will absorb IR radiation if the centers of positive and negative charge on the molecule change position as the molecule vibrates. This will be true for at least some vibrations of any molecule with three or more atoms. Any diatomic molecule that contains two different types of atoms will also absorb IR radiation, because the electronegativity of each of the two atoms of different elements are not the same. The only substances that will not absorb IR radiation are molecules that are composed of two atoms of the same element, called homonuclear diatomic molecules, or non-molecular substances, such as individual atoms and ionic substances that do not have polyatomic ions with covalent bonds. So, although not all two atom molecules are capable of being greenhouse gases, any molecule that has three or more atoms can act as a greenhouse gas.