Why is it o2 and not o?

and what does the 2 stand for?

Answer: Oxygen usually is in the form of diatomic molecules, this means each molecule contains two oxygen atoms.

Explanation: molecules form because of a lone oxygen atom is two electrons short of a full outer shell of eight. In order to fill the outer shell, one oxygen electron will share electrons with another one. Each oxygen atom 'gains' two electrons from the other. A total four electrons are shared between them, two from each atom, thus a double covalent bond is created.

Atoms

Explanation:

O2 is Written with a two as it is two oxygen atoms. Just o means one oxygen atom