Noble gas periods of the periodic table, the noble gases are exactly the members of group 18. Noble gases are typically highly unreactive except when under particular extreme conditions. The inertness of noble gases makes them very suitable in applications where reactions are not wanted. For example, argon is used in incandescent lamps to prevent the hot tungsten filament from oxidizing; also, helium is used in breathing gas by deep-sea divers to prevent oxygen, nitrogen and carbon dioxide (hypercapnia) toxicity. The properties of the noble gases can be well explained by modern theories of atomic structure: their outer shell of valence electrons
Photorespiration times more abundant, and in solution is 25 times more abundant than . The ability of RuBisCO to specify between the two gases is known as its selectivity factor (or Srel), and it varies between species, with angiosperms more efficient than other plants, but with little variation among the vascular plants. A suggested explanation into RuBisCO's inability to discriminate completely between and is that it is an evolutionary relic: The early atmosphere in which primitive plants originated contained very little oxygen, the early evolution of RuBisCO was not influenced by its ability to discriminate between and . Photorespiration rates are