Infographics / X-ray Physics

Atomic structure, shells & binding energy

Left: a Bohr-model atom with a nucleus of protons and neutrons surrounded by shells K, L and M at increasing radius; the K shell holds 2 electrons, the L shell 8. An L-shell electron drops into a K-shell vacancy and a characteristic X-ray photon is emitted. Right: an energy-level ladder showing binding energy greatest for the inner K shell and least for the outer shells; ionisation removes an electron to infinity (the full binding energy) whereas the characteristic photon energy equals the difference in binding energies between two shells.

Atomic structure: Bohr shells, binding-energy ordering, and characteristic X-ray production Left: a Bohr-model atom with a nucleus of protons and neutrons surrounded by shells K, L and M at increasing radius; the K shell holds 2 electrons, the L shell 8. An L-shell electron drops into a K-shell vacancy and a characteristic X-ray photon is emitted. Right: an energy-level ladder showing binding energy greatest for the inner K shell and least for the outer shells; ionisation removes an electron to infinity (the full binding energy) whereas the characteristic photon energy equals the difference in binding energies between two shells. Bohr model of the atom and characteristic X-ray production M L K nucleus (p + n) vacancy hν 0 · electron free (ionised) valence ≈ few eV M shell (outer) L shell K shell — deepest ≈ 69.5 keV (tungsten) binding energy increases ionisation = binding energy (to ∞) characteristic X-ray hν = BEₖ − BEₗ Nucleons = protons + neutrons. Z = number of protons. Shells fill K (2), L (8), M… up to 2n² electrons. Binding energy greatest for inner shells: K > L > M > valence; K-shell rises steeply with Z (≈ Z²). Ionisation removes an electron to infinity (= binding energy); excitation only lifts it by ΔE (< binding energy). Characteristic X-ray: outer electron fills an inner vacancy; hν = ΔE in binding energies (W Kα ≈ 59 keV).
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