On the periodic table, helium has the highest first ionization energy.
First ionization energy is the energy needed to remove one electron from a neutral atom in the gas phase. Helium’s value is about 2372 kJ/mol, the highest among the elements. Its two electrons occupy a very small, completely filled first shell that is strongly attracted to the nucleus.
Helium’s position at the top right of the periodic table helps explain this result. Across a period, effective nuclear attraction generally increases, while down a group, outer electrons are farther from the nucleus and more shielded. Helium combines the smallest occupied shell with a full shell, making electron removal especially difficult.
Neon also has a high first ionization energy, but it is lower than helium’s. Ionization energy should not be confused with electronegativity: fluorine leads the Pauling electronegativity scale, while helium leads first ionization energy.