We are constantly referring to the elements on the periodic table. SEVEN of those elements exist as diatomic molecules - meaning the formula is not just the symbol by itself, but the symbol-subscript 2. Here they are: name formula and state at 25 °C.
MEMORIZE this list! - memorize the names, formulas, and states of all seven of those. The diatomic formula is the correct formula for all of these. Anytime, you are to think single atoms, we will have to say single atoms. But if we say..."it then reacts with oxygen to make..." the oxygen we are referring to is O2.