The most difficult oxygen riddles

Singlet oxygen glows red as it decays from the singlet excited state to the triplet ground state : Singlet oxygen is utilised in organic syntheses, eg. Diels-Alder reaction. Ozone has a bluish colour.

St. Elmo's fire is a form of plasma. The electric field around the object in question causes ionization of the air molecules, producing a faint glow easily visible in low-light conditions. Conditions that can generate St. Elmo's fire are present during thunderstorms, when high voltage differentials are present between clouds and the ground underneath. The nitrogen and oxygen in the Earth's atmosphere cause St. Elmo's fire to fluoresce with blue or violet light; this is similar to the mechanism that causes neon lights to glow

Oxy-fuel welding was invented in 1903 by French engineers Edmond Fouché and Charles Picard. Pure oxygen, instead of air, is used to increase the flame temperature: a common propane/air flame burns at about 2,000 °C (3,630 °F), a propane/oxygen flame burns at about 2,500 °C (4,530 °F), and an acetylene/oxygen flame burns at about 3,500 °C (6,330 °F).

Oxygen has two valence electrons, so its valence is two. In water one oxygen molecule connects with two monovalent hydrogen molecules hence H2O.