How hard is it to pull one electron off an atom? 108 measured answers, one rhythm, seven rungs.
The first ionization energy is the energy needed to strip the outermost electron from a neutral atom in the gas phase. It is measured in electronvolts (eV). Hydrogen needs 13.6 eV.
Plot all of them against atomic number and the noble gases stand at the top of every period: He, Ne, Ar, Kr, Xe, Rn. A closed shell holds its electrons hardest. Helium, at 24.6 eV, holds tightest of all.
One step past each peak, the energy collapses. The alkali metals (Li, Na, K, Rb, Cs, Fr) carry one electron alone in a new shell, far from the nucleus and shielded by everything beneath. Caesium gives it up for about 3.9 eV.
Across the transition metals and the lanthanides the line barely moves. The electrons being added are going into inner d and f shells, so the outermost one feels almost the same pull each step.
Seven periods, seven rungs: the same climb and fall, again and again. This rhythm is the shell structure that the element spheres draw as rings. Past Z ≈ 108 there are no measurements: the atoms live for seconds or less, a few at a time.
LIT first ionization energies from the NIST Atomic Spectra Database (via the mendeleev data package), Z = 1–103. AMBER values for Z = 104–108 are calculated, not measured. WALL no values for Z = 109–118.
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