The octet rule

Group 18 of the periodic table contains the noble gases. They are the most stable elements on the periodic table because they have a complete outer shell of eight electrons. As a result, every other element will ionize and/or bond in a way that also leaves them with a complete outermost shell of electrons. This is known as the Octet Rule.

NB: Hydrogen bonds so that it completes its outer shell with two electrons, not eight.