Nitrogen can also bond with other three atoms because of its outer shell (valence shell) that has three electrons. This is the reason why organic molecules can be so large because of this bonding. Suppose you have a compound of NH3. You know that hydrogen can only share 1 electron because 7 of its electrons are filled. Also, in nitrogen, it can only share 3 electrons because 5 of it are already filled. That is why nitrogen needs three hydrogen to form NH3.