The nitrogen and hydrogen atoms that make up ammonia are held together by

Nitrogen and hydrogen atoms that make up ammonia (NH3) are held together by polar covalent bonds, where electrons are shared unequally. Nitrogen is significantly more electronegative than hydrogen, so shared electrons spend more of their time orbiting the nitrogen atom.

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom.