Here's why:

* H2 represents a diatomic molecule of hydrogen, meaning two hydrogen atoms bonded together. This is the most common form of hydrogen found in nature.

* Hydrogen can exist as a single atom, denoted as H. This is known as atomic hydrogen and is less stable than the diatomic form.

* Hydrogen can also form ions. For example, a positively charged hydrogen ion is written as H+.

Therefore, it's important to consider the context to understand whether hydrogen is represented as H2, H, or another form.