4. why can't a third hydrogen atom join the h, molecule to make h3?

Hydrogen is diatomic.

Explanation:

Because hydrogen exists as a diatomic molecule.

In the hydrogen molecule, the electrons from each atom are able to be near two protons instead of only the one proton it started with. Whenever negative electrons are near additional positive protons, the arrangement is more stable.

Hope this helps ;)

that tool would be a compass