The gaseous chemical element with symbol O that is essential in the respiratory processes of most living cells and in combustion processes. It’s the most abundant element in The Earth’s crust. Nearly one fifth of the air’s volume is made of oxygen. Non-combined gaseous oxygen normally exists in form of diatomic molecules, O2, but it also exists in triatomic form, O3, named ozone. In normal conditions oxygen is a colourless, odourless and insipid gas; it condensates in a light blue liquid.