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In 1913 Henry Moseley showed that the X-rays an element gives out climb steadily in frequency with its atomic number (Moseley, Philosophical Magazine, 1913). He had photographed just two lines in the K group and four in the L group (Nobel presentation speech, 1925). Over the next ten years, Manne Siegbahn in Sweden turned those six lines into a detailed map of the atom's inner levels.
Who was Manne Siegbahn?
Karl Manne Georg Siegbahn was born on 3 December 1886 in Örebro, Sweden, and died on 26 September 1978 (Nobel Prize biography). At the University of Lund he assisted J. R. Rydberg, took his doctorate in 1911 and became professor in 1920, moving to Uppsala in 1923. From 1912 to 1937 his research was mainly devoted to X-ray spectroscopy, the same biography says.
The Nobel Prize in Physics 1924 was awarded to him "for his discoveries and research in the field of X-ray spectroscopy" (Nobel Prize facts). He received it a year later, in 1925.
What did Siegbahn discover in X-ray spectra?
X-ray spectroscopy means splitting X-rays into their separate wavelengths, usually by reflecting them off a crystal whose evenly spaced atoms act like a grating (Nobel presentation speech). Each element gives out its own sharp lines, its characteristic X-rays.
Over ten years Siegbahn improved or redesigned almost every detail of his apparatus, the presentation speech says. His biography adds that his spectrographs were held at a constant temperature and read in tenths of a second of arc (Nobel Prize biography). In the speech, Allvar Gullstrand, chair of the Nobel Committee for Physics, said that "the exactitude with which wavelengths can now be measured by his methods is a thousand times greater than that attained by Moseley" (Nobel presentation speech).
That precision led to discoveries. The speech lists what his group had measured by 1925:
K series: studied in 42 elements, with all four main lines measured for 27 of them.
L series: 28 lines, studied in about 50 elements.
M series, which Siegbahn discovered: 24 lines in 16 elements.
N series, discovered under his guidance: found in three of the heaviest elements, with five lines measured for uranium and thorium.
He collected the results in his book Spektroskopie der Röntgenstrahlen (1923; English edition 1924), and his biography notes that his energy-level values still served for many purposes some thirty years later (Nobel Prize biography).
How did X-ray spectra prove atomic shell structure?
The presentation speech sets out the reasoning in three steps (Nobel presentation speech):
K lines have shorter wavelengths, so higher frequencies, than L lines from the same element.
A photon's energy is proportional to its frequency, so a K line comes from a bigger energy drop.
So the level an electron falls into to make a K line must lie closer to the nucleus. Outside it come L, then M, then N.
Siegbahn's exact measurements let other physicists go further: there are three different L levels, five M levels and seven N levels, the speech says. The NIST X-ray transition database lists copper's Kα1 line under the name KL3, a transition between the K level and the third L level (NIST X-ray Transition Energies).
For the backstory, read our posts on Charles Barkla and characteristic X-rays and Henry Moseley and the atomic number.
Does Moseley's law predict copper's X-ray line?
In modern form, Moseley's law for the Kα line is E = (3/4) × 13.6 eV × (Z − 1)² (Moseley's law, Wikipedia). The 13.6 eV is the Rydberg energy, whose CODATA recommended value is 13.605693 eV (NIST CODATA). The "− 1" is the shell picture at work: in a simplified explanation, the one electron left in the K level screens part of the nucleus's charge.
Try it for copper, Z = 29:
Z − 1 = 28, and 28² = 784.
(3/4) × 13.6057 eV = 10.204 eV.
E = 10.204 eV × 784 ≈ 8,000 eV.
NIST's measured Kα1 energy for copper is 8,047.82 eV (NIST X-ray Transition Energies).
The simple law is off by about 48 eV, or 0.6%. With the same NIST table, iron (Z = 26) gives 6,378 eV against 6,404 eV measured, about 0.4% low, and molybdenum (Z = 42) gives 17,153 eV against 17,479 eV, about 1.9% low. Rough measurements hide such gaps; precise ones, like Siegbahn's, show where the model needs fixing.
For more practice, see our explainer on X-ray wavelengths and Moseley's law. If shells and subshells still feel shaky, the Fermi Concept Library gives each concept in physics and chemistry its own lesson. And when your Moseley numerical goes wrong, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
What can students learn from Siegbahn?
Siegbahn did not propose a new theory of the atom. He measured carefully for 25 years, and the Nobel speech predicted his results would long remain the touchstone for changes in atomic physics. When your answer lands 0.6% from the data, ask what the gap is telling you.
