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Chemistry

Ionization Energy: The Energy Cost of Removing an Electron

Quick fact

Caesium, with its single valence electron, has one of the lowest ionization energies of any element—so low that it can be ionized by visible light. In contrast, helium, with a filled shell, requires the highest ionization energy of all elements.

Why this is interesting

Why is it easier to pull an electron off a huge atom like cesium, but nearly impossible to pull one off a tiny atom like helium? The answer lies in the energy of a single electron.