system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why cant atoms like hydrogen and chlorine have full shells and not make a molecule of 2??

What I mean is, why cant chlorine gain an electron to get a full shell without making Cl2 molecules?? Why cant it exist as a single atom?? The same with hydrogen, why does it have to form H2 molecules?? If the element has a full sub shell and is stable, isnt that enough?
gpt
There is nothing that says they can't. But that would only likely happen in a vacuum. The electron they have had to come from somewhere. In your examples, the two Cl and H atoms are basically sharing electrons in a covalent bond.

Even if you had a free electron to give them, they would become ions, meaning they would have an electric charge, so would be attracted to positively charged ions elsewhere, forming ionic bonds.