Why is the photodissociation of \(\mathrm{N}_{2}\) in the atmosphere relatively unimportant compared with the photodissociation of \(\mathrm{O}_{2}\) ? Human Activities and Earth's Atmosphere (Section 18.2)

Photodissociation is a process in which a molecule absorbs a photon of light, which causes the molecule to break apart into its constituent atoms or smaller molecules. In the context of the atmosphere, photodissociation of N2 and O2 is important as it results in the formation of reactive species that play a role in various atmospheric processes.

The energy required to break the bond in a molecule can be represented as the bond dissociation energy. In order to break the bond, the molecule must absorb a photon with at least equal energy to the bond dissociation energy. For N2, the bond dissociation energy is around 9.8 eV, while for O2, it is around 5.2 eV. Since N2 has a stronger bond, it requires more energy to be photodissociated compared to O2.

The energy of a photon is inversely proportional to its wavelength. Since the photodissociation of N2 requires more energy, the corresponding photons must have shorter wavelengths. The photons with enough energy to photodissociate N2 are mostly in the ultraviolet (UV) range, particularly the extreme UV range below 200 nm. However, these photons are not very abundant in the Earth's sunlight, as they are mostly absorbed by other molecules or scattered before reaching the Earth's surface. On the other hand, O2 absorbs photons in a wider range of wavelengths, with the energy requirement met by UV-B (280-320 nm) and UV-C (100-280 nm) photons. These wavelengths are more plentiful in sunlight and can penetrate deeper into the atmosphere.

When N2 undergoes photodissociation, it forms nitrogen atoms (N) which are relatively unreactive and do not cause any significant impact on the atmosphere. On the other hand, photodissociation of O2 forms reactive oxygen atoms (O) which further react with other O2 molecules to form ozone (O3). This ozone plays a crucial role in absorbing UV radiation and protecting the Earth's surface from harmful UV radiation. Moreover, ozone is also involved in various other chemical reactions in the atmosphere contributing to the Earth's weather and climate.

The photodissociation of N2 is relatively unimportant compared with the photodissociation of O2 in the atmosphere due to three main reasons: (1) higher energy requirements for N2 photodissociation, (2) the scarcity of short-wavelength UV photons needed to break the N2 bonds, and (3) the lack of any significant atmospheric consequences resulting from the photodissociation of N2 compared to the various roles played by O2 and its photodissociated products like ozone.