Stick with me, I promise this will all make sense in a little bit.
This story starts with diatomic elements like oxygen and hydrogen — not quite the opening you were expecting, I know. Oxygen and hydrogen are diatomic (i.e., they consist of two individual atoms bound together) naturally in their elemental state. They do not exist as single atoms naturally like helium or neon. This seems like a relatively inconsequential fact, but historically, this gave early chemists the biggest pain in the ass.