Whether a gas is a greenhouse gas depends on whether its vibrational modes can absorb/emit infrared radiation, which in turn depends on whether those vibrations change the molecule's dipole moment; homonuclear diatomic molecules like N2 and O2 have only one vibrational mode that is IR-inactive by symmetry.
N2 and O2 are homonuclear diatomic molecules with only a single vibrational degree of freedom (the bond-stretch mode), and because this vibration does not change...
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