Why do greenhouse gases trap infrared radiation but not homonuclear molecules?

Answer 1

1) Absorption of IR radiation depends on the dipole moment of a molecule (which might be considered the tension on the shared electrons within the molecule).

2) In a homonuclear molecule (such as O2), the identical nuclei exert an identical pull on the shared electrons. The dipole moment is zero, and can interact with radiation of zero frequency and zero wavelength. Such radiation does not exist.

3) In a heteronuclear molecule such as water, the differing nuclei of oxygen and hydrogen exert an unequal pull on the shared electrons. This produces a non-zero dipole moment which is capable of interacting with infrared radiation, raising the molecule to a higher energy level.

4) Carbon dioxide (CO2) is a particular case. The oxygen atoms are at exactly opposite sides of the carbon. Although each side has a dipole moment, since the molecule is symmetrical it tends to cancel out.

However, there is the possibility of movement of nuclei within the molecule. If the movement is symmetrical, there is no dipole moment. If the movement is asymmetrical, a dipole moment is temporarily produced. If there is infrared radiation present in the right orientation, interaction is possible. Therefore carbon dioxide is a fairly weak greenhouse gas.

However, since it is being continually introduced into the atmosphere by human activity, its effect is being raised continually as well.