Oxygen’s three major forms are dynamically interrelated: molecular oxygen (O2), atomic oxygen (O), and ozone (O3). Each form has a more or less dominant role in different layers of the atmosphere: O2 in the troposphere (< 8 km over the Poles to > 16km over the Equator), O3 in the stratosphere (< 50 km), and O in the exosphere (> 500 km) where most O2 and O3 are photo-dissociated by ultraviolet radiation. About 90% of Earth’s atmospheric mass, estimated at 5.2 x 1021 g, of which 1.18 x 1021 g is oxygen, is located < 15 km above its surface. Gravity, temperature, radiation, and planetary rotation influence oxygen distribution variously at different atmospheric levels.*
Oxygen’s chemical reactivity is determined by the unpaired electron’s spin pattern in O’s outer orbital. When two Os form O2 the parallel spin pattern of each outer orbital electron is maintained even as the electrons are shared. This anomaly confers relative stability to ground state O2. Because the Pauli exclusion principle holds that anti-parallel spin electrons must pair with parallel spin electrons, the two parallel spin electrons occupying O2’s outer domain do not readily react with other moieties at the same time. Energy must be expended to overcome this electron restriction to produce spin reversal for O2 to react. Thus, reduction of O2 to H2O progresses stepwise via energy-dependent serial electron transfers that create distinct reactive intermediaries with different half-lives, diffusion capabilities, biological significance, and toxicity.
It is this oxygen property related to outer orbital electron spin restriction that frames the “oxygen paradox,” defined here as obligate aerobic life-form dependency on a chemical reduction process that produces invariably harmful intermediaries hazardous to the very life forms that produce them. Implicit in the oxygen paradox is another paradox – free radical (FR) production amounts to self-generated “inner radiation,” protection from which is conferred by intrinsic antioxidant capacity.
*The technically minded reader may want to review or read:
Stepniewski W, Stepniewski Z, Bennicelli RP, Glinski J. Oxygenology in Outline. Institute of Agrophysics PAS, Lublin 2005
