You are good at Science, as well as at English! Yes, the letter A is also a word.
Just like most letters don't form words on their own, most atoms don't exist on their own. For example, we said that Oxygen (O) is found in the atmosphere. But, Oxygen won't be found in the form of individual atoms.

Atoms combine with other atoms to form molecules. For example, one O combines with another O atom to form the Oxygen molecule. This is similar to multiple letters combining to form a word.