Hydrogen atom has only one valence shell( 1-s orbital) which can accommodate only 2 electrons. Also hydrogen atom has only 1 proton so if ever it gained 8 electrons, it would have a net charge of -7 which will make hydrogen atom highly unstable. Octet rule applies perfect only upto atomic number- 20...after that this rule doesnt works cuz of extended octet. Hence hydrogen is an exception to octet rule.
