Which element has the largest ionization energy?
Helium has the largest ionization energy.
Sodium has the greatest ionization energy of the four elements listed from column 1 of a wide form periodic table. Among this group of metals that readily form cations, the largest always has the lowest ionization energy and the smallest has the most. This is generally ascribed to the fact that the valence shell electron is further from the nucleus in the largest element and nearest in the smallest element.
First ionization energy is the energy required to remove one electron from an atom. Second ionization energy is the energy required to remove a second one, and third, well, you get the picture. An ionization energy chart can help you determine what group an element is in. If an element has a relatively low first ionization energy, but then there's a huge jump in ionization energy to second, then it's probably a group 1 element…