Question

Why is oxygen written as ${O_2}$ ?

Answer 1

Because pure oxygen is highly reactive, it will combine with anything to produce oxides, and unconnected atoms have a strong inclination to form bonds, it is available as ${O_2}$ rather than $O$ . So, if an oxygen atom has nothing else to unite with, it will join with another oxygen atom to produce an oxygen molecule.

Complete answer:
The distinction between oxygen $\left( O \right)$ and oxygen $\left( {{O_2}} \right)$ is that the former is an oxygen atom, whilst the latter is a molecule made up of two $O$ atoms bonded together. The most common form of oxygen is as a diatomic gas. As a result, we refer to it as ${O_2}$ .
According to Avogadro, hydrogen and oxygen are diatomic molecules. If this is the case, the following is the reaction that results in the production of water:
$2{H_2} + {O_2} \to 2{H_2}O$
In other words we can also say that, when there are two oxygen atoms in a molecule, it is denoted as ${O_2}$ . Individual atoms of oxygen do not usually exist; instead, two oxygen atoms bind together to form an oxygen molecule. Due to the high reactivity of oxygen, unconnected atoms have a great inclination to form bonds. They'll bond to each other if there's nothing else available.
Chemical equations are constructed in such a way that they show all of the molecules involved. Because each oxygen molecule has two oxygen atoms, it is written with a $2$ .

Note:
We breathe diatomic oxygen molecules, which are made up of only two oxygen atoms. Diatomic oxygen is a relatively stable oxygen molecule that accounts for roughly $21\%$ of the air we breathe.