1. Oxygen has the atomic number 8.
2. Its chemical symbol is O.
3. At standard temperature and pressure two atoms of the element bind to form dioxygen, a colourless, odourless, tasteless diatomic gas with the formula O2.
4. After hydrogen and helium, oxygen is the third most abundant element by mass in the universe.
5. It makes up about 21% of the volume of our atmosphere on Earth.
6. Oxygen is highly oxidizing. It readily accepts electrons from other substances during reactions. Oxygen reacts vigorously with combustible materials, especially in its pure state, releasing heat in the reaction process.
7. Oxygen has a low boiling/ condensing point: -297.3°F (-183°C). The gas is approximately 1.1 times heavier than air and is slightly soluble in water and alcohol.  Below its boiling point, oxygen is a pale blue liquid slightly heavier than water.
8. Swedish pharmacist Carl Wilhelm Scheele is said to have discovered a gas he called ‘fire air’ around 1772 by heating mercuric oxide and various nitrates. What he found was oxygen, the only known supporter of combustion.
9. In 1774, two other scientists claimed to have independently discovered the gas. Joseph Priestley, a British clergyman is normally given credit for the discovery because he published his findings before Scheele. Also, the French chemist, Antoine Laurent Lavoisier claimed he had discovered the gas before Scheele. Although that may not be true, Lavoisier was the first to prove that oxygen was a chemical element and describe how combustion works.
10. The World’s oceans carry dissolved oxygen that supports life. The polar oceans, being coolest, hold more dissolved oxygen and therefore sustain vast amounts of aquatic life.
11. Photosynthesis releases oxygen into the atmosphere, while respiration and decay remove it from the atmosphere.
12. Green algae and cyanobacteria in marine environments provide about 70% of the free oxygen produced on Earth through photosynthesis and the rest is produced by the terrestrial plants.
13. Here’s the simplified chemical equation that describes photosynthesis:
6CO2 + 6H2O + photons → C6H12O6 + 6O2
carbon dioxide + water + sunlight → glucose + dioxygen
14. Molecular dioxygen, O2, is essential for cellular respiration in all aerobic organisms. Oxygen is used in mitochondria to help generate adenosine triphosphate (ATP) during oxidative phosphorylation. The reaction for aerobic respiration is essentially the reverse of photosynthesis and is simplified as:
C6H12O6 + 6O2 → 6CO2 + 6H2O + 2880 kJ·mol-1
glucose + dioxygen → carbon dioxide + water + energy
15. Ozone (O3) or trioxygen, is an allotropic form of oxygen that is more reactive than ordinary oxygen. Ozone is formed in nature by electrical discharges or by irradiation with ultraviolet light in the Earth’s upper atmosphere. In the upper atmosphere it acts as a protective shield, protecting us from UV. However, produced as a by-product of automobile exhaust, it’s a pollutant closer to the surface of the planet. It is harmful to lung tissue. It’s sometimes used in the treatment of drinking water instead of chlorine.
16. Oxygen is obtained for commercial and industrial uses through cryogenic distillation of liquefied air.
17. Water (H2O), or hydrogen oxide is the most familiar oxygen-containing compound. Others oxides include iron oxide (Fe2O3) or rust; carbon dioxide (CO2); aluminium oxide (Al2O3) and silica (SiO2).
18. Oxygen is also present in many organic compounds: carbohydrates, fats, fatty acids, amino acids, proteins, ethers, esters, alcohols, keytones, aldehydes, carboxylic acids… to name just a few.
19. Too much oxygen can be toxic. This can happen when scuba diving for example, if the lungs take in a higher than normal 02 partial pressure.
20. Oxygen is said by many to be a mild euphoric, but its supposed performance-enhancing effects in sports such as athletics is not medically proven.