Oxygen: The Breath of Life

Oxygen fuels respiration and combustion alike, reacting with almost every element on the periodic table. Photosynthetic microbes flooded the atmosphere with it over two billion years ago in the Great Oxidation Event. Liquid oxygen is pale blue and magnetic, and the ozone layer shields surface life from ultraviolet radiation. Too little oxygen kills, yet too much is toxic, so biology keeps it finely balanced..

.