Oxygen is a non metallic chemical element of the periodic table. Oxygen is colourless and odourless. It is a tasteless gas essential to living organisms. Animals convert it to carbon dioxide and plants in turn utilize carbon dioxide as a source of carbon and return the oxygen to the atmosphere. Oxygen forms compounds by reaction with practically any other element as well as by reactions that displace elements from their combinations with each other. In many cases these processes are accompanied by the evolution of heat and light and in such cases are called combustions. Its most important compound is water. Oxygen was discovered by a Swedish chemist Carl Wilhelm Scheele who obtained it by heating potassium nitrate and mercuric oxide along with many other substances. An English chemist Joseph Priestley independently discovered oxygen in by the thermal decomposition of mercuric oxide and published his findings the same year which was three years before Scheele published. French chemist Antoine with remarkable insight interpreted the role of oxygen in respiration as well as combustion discarding the earlier theory which had been accepted up to that time. He noted its tendency to form acids by combining with many different substances and accordingly named the element oxygen from the Greek words for acid former. Oxygen is the most plentiful element in Earth crust. The proportion of oxygen by volume in the atmosphere is one third and by weight in seawater is nine tenth. In rocks it is combined with metals and non metals in the form of oxides that are acidic or basic and as salt like compounds that may be regarded as formed from the acidic and basic oxides such as sulphates and carbonates or aluminates and phosphates. Plentiful as they are these solid compounds are not useful as sources of oxygen because separation of the element from its tight combinations with the metal atoms is too expensive.