The ionization energy of an element is the amount of energy needed to rip an electron from an atom in the gaseous state. The ionization energy increases going up and to the right of the periodic table.
The ionization energy increases going from the left to the right of the periodic table because elements toward the right of the table have smaller radii; the higher number of protons in the nucleus exert a greater force on the orbiting electrons, pulling them closer to the nucleus. Ionization energies increase from the bottom of the periodic table to the top because smaller atoms hold onto their electrons more tightly than bigger atoms. The reasoning behind this concept is that in bigger atoms, electrons orbit farther away from the nucleus; therefore, the protons in the nucleus exert less of an attractive force on the electrons. According to these general rules, helium is the element with the highest ionization energy.