Oxygen

Oxygen is the 8th element in the periodic element according to its atomic number. It is the second most electronegative element after Fluorine. Its electronegativity is 3.44 according to Pauling Scale. 
It is also a powerful oxidising agent having formed many oxides with elements readily. 
Under standard temperature and pressure it exists as dioxygen(O2) which is a colorless, odourless and tasteless gas. 
Free elemental O2 only began to accumulate in the atmosphere about 2.5 billion years ago during the Great Oxygenation Event, about a billion years after the first appearance of the Obligate anaerobic organisms.
Category:Element