The density of oxygen at standard temperature and pressure is 1.429 g·L-1. Oxygen has the atomic number 8. It's a very reactive gas that combines with a great variety of other elements.
These combinations are called oxides. When a molecule of oxygen combines with two molecules of hydrogen, it forms water, which is essential to human life. Oxygen is also part of the air that makes up Earth's atmosphere.
There are two allotropes of oxygen. O2 is a colorless gas, and O3 is ozone, a blue gas with a distinctive, sharp smell. Although ozone is poisonous, Earth's ozone layer protects it from the sun's damaging ultraviolet rays.