Atmospheric oxygen is denoted by what symbol?

##O_2(g)##

Oxygen as a gas is made up of molecules that each are built from two atoms of the element oxygen, that is denoted by ##O##.

There is another form of oxygen, known as ozone ##O_3##, that has three oxygen atoms per molecule. Though highly poisonous, it is an essential part of the upper atmosphere (it stops dangerous UV rays).