Answer:
helium
Explanation:
The ionization energy decreases from top to bottom in groups, and increases from left to right across a period. Thus, helium has the largest first ionization energy, while francium has one of the lowest.
Helium has higher first ionization energy because the electrostatic forces of attraction between the nucleus (+2e) with the electron (-1e) are apparently higher than those between the H nucleus (+1e) and the electron (-1e) (by Coulomb's law), so is the first ionization energy.
