Elements With the Highest First Ionization Energy

About This Dataset

This chart ranks elements by first ionization energy, the energy in kilojoules per mole needed to remove one electron from a neutral gaseous atom. Higher values mean the atom holds its electrons more tightly. Helium leads.

✨ AI Insight

Helium requires the most energy to ionize, 2,372 kJ/mol, because its two electrons occupy a compact, fully filled shell tightly bound to the nucleus. Neon (2,081) follows, and the pattern is clear: noble gases dominate the top of the list thanks to their exceptionally stable electron configurations. Fluorine (1,681) is the highest non-noble-gas element, consistent with its record electronegativity. The values put the periodic law on display, ionization energy generally rises across a period and falls down a group, which is why light elements like helium, neon and fluorine outrank heavier ones such as krypton and xenon. Even hydrogen, with a single electron, needs 1,312 kJ/mol, more than chlorine, underscoring how tightly the smallest atoms bind their electrons.

Source: Ionization energies of the elements

Key Findings

Auto-computed highlights from this dataset

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Helium leads with 2,372 in All Time.

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Average value in All Time: 1,546 · median: 1,377.

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Gap in All Time: Helium is 2× larger than Xenon.