Helium and hydrogen have almost the same atomic size, yet the ionization energy of helium is almost twice that of hydrogen. explain why hydrogen has a much higher ionization energy than any element in group 1 does.

Ionization energy is the total energy needed to eliminate an outermost electron. Since Helium is a noble gas - the electrons are nearer to the nucleus while in hydrogen the electrons are further away from the nucleus. Since the space between hydrogen nucleus and the electrons are bigger it is at ease to remove an electron as different to helium which has a shorter distance between the nucleus and electrons.