ATOMIC NUMBER: The atomic number of Oxygen is 8.
ATOMIC WEIGHT: The standard atomic weight of Oxygen is 16.
NB - please see also: relative atomic mass, atomic mass, atomic weight, elements, isotopes, and periodic table [all linked to the right].
Oxygen is the most abundant element in the Earth's crust where it is found combined with other elements, mainly in the form of oxides.
At standard conditions, oxygen is a gas. Atoms of oxygen are combined in pairs to form oxygen molecules, chemical formula O2, which make up approximately 21% of the Earth's atmosphere.
High up in the atmosphere, UV light energy from the sun breaks the chemical bonds between the atoms in the oxygen molecules, and some of the free atoms react with unbroken O2 to form ozone molecules, chemical formula O3. The ozone thus formed is known as the ozone layer.