Three hydrogen atoms are necessary to join with one nitrogen atom to form a stable compound. This compound is ammonia (NH₃).

Here's why:

* Nitrogen's Valence: Nitrogen has 5 valence electrons (electrons in its outermost shell). It needs 3 more electrons to achieve a stable octet (8 electrons).

* Hydrogen's Valence: Hydrogen has 1 valence electron. It needs 1 more electron to achieve a stable duet (2 electrons).

By sharing electrons through covalent bonds, one nitrogen atom can share its electrons with three hydrogen atoms, forming ammonia.