- m.p. -229 deg.C, b.p. -183 deg.C

- ¹⁶O 99.56% I = 0; ¹⁸O 0.4% ; ¹⁷O 0.04% I = 5/2

 - 20.9% by volume of Earth's atmosphere.

 - Most abundant element (46.6%) in Earth's crust.

 - Essential for life.

 - Two Allotropes: O₂ and O₃

* Ground state of O₂ is a triplet S = 1

 * Excited form of O₂ with paired p* electrons (singlet) ¹  D_g state

 - red chemiluminesence 95 kJmol^-1 above ground triplet state

 - from H₂O₂ + OCl^-  Cl^- + H₂O + O₂

 ^{- reactive, especially with organic compounds:} 

MO Diagram for O₂

 Figure 1: The molecular orbital diagram of dioxygen.