Henry Moseley

How to use this course

Henry Moseley’s research career lasted from the autumn of 1910 to the summer of 1914. In that time he learned radioactivity in Ernest Rutherford’s Manchester laboratory, took up the new subject of X-ray diffraction within months of its discovery, and then measured the X-ray spectra of some forty elements. He found that the square root of the frequency of a characteristic X-ray line rises by the same step from one element to the next. The step counts the charge on the nucleus. Atomic number, until then just a position in a table, became a measurable physical quantity.

This course follows his physics in the order he did it. Each result is derived with modern tools and set beside the evidence Moseley actually had. Readers who have taken the companion courses on Ernest Rutherford and J. J. Thomson will recognize the nuclear atom of 1911 and the ionization chamber.

Each technical lesson opens with a paragraph for the general reader, then gives precise statements, derivations and exercises with folded solutions. Lesson 07, Moseley’s law and its explanation by the Bohr model, is the centre of the course. Lesson 08 shows what the law did to the periodic table.

Timeline

Weymouth, Oxford and Eton, 1887–1906

Henry Gwyn Jeffreys Moseley was born on 23 November 1887 in Weymouth on the Dorset coast. Both sides of his family were scientists. His father, Henry Nottidge Moseley, was a naturalist who had sailed on the Challenger expedition of 1872–76, the first great oceanographic voyage, and who became professor of human and comparative anatomy at Oxford. His grandfather, Henry Moseley, was a mathematician, a professor at King’s College London and a Fellow of the Royal Society. His mother, Amabel, was the daughter of the conchologist John Gwyn Jeffreys. She later became British women’s chess champion, in 1913.

His father died in 1891, when Henry was three. His mother brought up the children in and around Oxford. Many years later she married the Oxford geologist William Johnson Sollas.

School

He went to Summer Fields, a preparatory school in Oxford, and then to Eton College as a King’s Scholar. At Eton he concentrated on mathematics and science. In his last year, 1906, he won the school prizes in both chemistry and physics.

Trinity College, Oxford, 1906–1910

In 1906 Moseley entered Trinity College, Oxford, with a Millard Scholarship. He took a first in Mathematical Moderations, the preliminary examination in mathematics, and then read natural science with physics as his subject. His final degree in 1910 was a second, a disappointment to him, though nobody who later worked with him took it as a measure of his ability. While at Oxford he joined the Apollo University Lodge of Freemasons.

Physics at Oxford was then weak in research. Moseley wanted to work on the new physics of the atom, and the place for that in Britain was Manchester, where Ernest Rutherford had arrived in 1907 and had already been awarded the 1908 Nobel Prize in Chemistry. Moseley asked to join Rutherford’s laboratory and was accepted.

Manchester, 1910–1913

Moseley joined the University of Manchester in September 1910 as lecturer and demonstrator in physics. Teaching took much of his first year, after which he moved to research. From 1912 he held part of the John Harling Fellowship, which paid the same salary as his demonstrator post but freed him from teaching. He was elected to the Manchester Literary and Philosophical Society in May 1911.

Rutherford’s laboratory was the world centre of radioactivity. In 1911 Rutherford published the nuclear model of the atom. Hans Geiger and Ernest Marsden were measuring the scattering of alpha particles; Charles Galton Darwin, grandson of the naturalist, was the theorist of the group; Kazimierz Fajans and George de Hevesy were among the radiochemists. Niels Bohr arrived in March 1912 and stayed until July, working out the ideas that became his 1913 model of the atom. Moseley started with radioactivity (Lesson 04).

The turn to X-rays

In 1912 Max von Laue, Walter Friedrich and Paul Knipping in Munich showed that crystals diffract X-rays. Moseley and Darwin at once decided to study the effect, although nobody in Manchester knew the techniques. William Lawrence Bragg in Cambridge and his father William Henry Bragg at Leeds were moving faster. Moseley visited Leeds to learn their methods. By the summer of 1913 he and Darwin had mapped the X-ray spectrum of a platinum target (Lesson 05). Moseley then set out on his own to measure the spectra of many elements (Lessons 06–08).

In the spring of 1913 Rutherford offered him the full John Harling Fellowship. He declined it. Late in 1913 he moved back to Oxford to live with his mother and continue his research there (Lesson 11).

Apprenticeship in radioactivity

For the general reader. Every new member of Rutherford’s laboratory began on radioactivity. Moseley’s first projects were small and hard. He counted the electrons thrown out by decaying radium products, chased radioactive species that lived for only a few thousandths of a second, and built a device in which a speck of radium charged itself to more than a hundred thousand volts. It was the first nuclear battery. These projects taught him vacuum technique and precise measurement, the skills his X-ray work would need.

Short-lived products

In 1911 Moseley and Fajans studied the short-lived products of actinium and thorium. They found that a product of actinium, then called actinium A, was half transformed in about 1/500 of a second. Today it is known as polonium-215, with half-life 1.78 ms. Rutherford, in his obituary of Moseley, credited him with devising the method.

Any method for such short lives must separate the time of formation from the time of measurement by a known and adjustable interval. One way is to carry freshly formed atoms on a moving surface past a detector. If the surface moves at speed \(v\) and the detector sits a distance \(x\) downstream, the delay is \(t=x/v\).

The radium battery

Radium in equilibrium with its products emits beta particles, which are fast electrons. If the source sits on an insulated support in a vacuum, each beta particle that escapes leaves a positive charge behind. In 1913 Moseley published The attainment of high potentials by the use of radium. His radium source, in a high vacuum, charged itself to more than 100 000 volts and held that potential for weeks.

The second limit is fundamental. The beta particles of radium C have energies up to about 3 MeV, so no potential much above a few million volts can be reached this way, however good the insulation. The principle survives in modern betavoltaic and direct-charge nuclear batteries.

X-ray diffraction and Bragg’s law

For the general reader. X-rays were discovered in 1895, but for seventeen years nobody could tell whether they were waves or particles. In 1912 Laue showed that a crystal acts on X-rays as a ruled grating acts on light, because its atoms are spaced at about the X-ray wavelength. The Braggs turned this into a simple rule: a crystal reflects X-rays of a given wavelength only at certain angles. Measure the angle and you know the wavelength. Moseley and Darwin used the rule to map the spectrum of a platinum target, and it became Moseley’s ruler.

The Bragg condition

Think of a crystal as a stack of parallel atomic planes a distance \(d\) apart. An X-ray beam meets them at glancing angle \(\theta\), measured from the planes. Each plane reflects a small fraction of the beam. Rays reflected from neighbouring planes differ in path by \(2d\sin\theta\). The reflections add in phase only if that difference is a whole number of wavelengths.

Because the reflections come from thousands of planes, the peaks are sharp, just as many slits make a grating’s lines sharp. Laue’s original description, in terms of three-dimensional interference from a lattice, gives the same condition. Laue received the Nobel Prize in Physics for 1914 and the Braggs for 1915.

Moseley and Darwin, 1913

Moseley and Darwin built an X-ray spectrometer with an ionization chamber on a rotating arm. They expected the ionization method to give better quantitative measurements than photography. Reflecting the radiation from a platinum target off rock-salt and potassium ferrocyanide crystals, they found a continuous background on which sharp peaks stood at fixed angles. The peaks were the characteristic lines of platinum. William Henry Bragg found the same lines independently at Leeds. Their joint paper, The reflexion of X-rays, appeared in the Philosophical Magazine in July 1913. Darwin went on to write the first theory of the intensity of X-ray reflection from a perfect crystal, the beginning of dynamical diffraction theory.

The scale of the ruler

To turn angles into wavelengths one needs \(d\). For rock-salt, a cubic crystal with sodium and chlorine ions on alternating sites, each small cube of side \(d\) contains half a molecule of NaCl on average. So \(\rho d^3=M/(2N_A)\), with \(M\) the molar mass, \(\rho\) the density and \(N_A\) Avogadro’s number. Moseley calibrated his potassium ferrocyanide crystal against rock-salt in exactly this way.

Characteristic X-rays and the spectrometer

For the general reader. When fast electrons hit a metal they produce X-rays of two kinds: a smooth spread of wavelengths, and a few sharp lines that depend only on the metal. Charles Barkla had found the lines by absorption methods and named the two groups K and L. Moseley built an X-ray tube with a little trolley inside that carried samples of different metals into the electron beam one after another, and photographed the lines of each through a crystal. The result was a picture in which the lines step steadily across the plate from one element to the next.

Two kinds of X-ray

Electrons accelerated through a potential \(V\) decelerate in the target and radiate a continuous spectrum, called bremsstrahlung. No photon can carry more energy than one electron brings, so the spectrum ends sharply at a shortest wavelength.

On top of the continuum stand the characteristic lines. Barkla, working from 1906 with secondary radiation from many elements, had found that each element emits a harder group, which he called K, and a softer group, L. He measured them by their absorption in aluminium, not by wavelength. Bragg spectrometry made precise wavelength measurement possible.

The 1913 apparatus

Moseley’s tube was a long glass vessel pumped out by a Gaede mercury pump. Inside, the target elements were mounted on a small trolley which he could pull along a rail with a magnet from outside, so each target could be brought under the cathode beam without opening the vacuum. A fine slit in a platinum plate sat before the bombarded spot. The X-rays left through a thin aluminium window, struck a potassium ferrocyanide crystal on a spectrometer table, and fell on a photographic plate. With slit and plate at equal distances from the crystal axis, the position of a line on the plate did not depend on which part of the crystal the beam hit. Exposures were about five minutes.

Each element gave two main K lines, a strong one Moseley labelled α and a weaker one, β, at shorter wavelength. His famous photograph shows the lines of calcium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper and brass stepping across the plate. Brass showed both the copper and the zinc lines, and the cobalt sample showed a nickel impurity line. The paper, The high-frequency spectra of the elements, appeared in the Philosophical Magazine in December 1913.

Moseley’s law

For the general reader. Moseley noticed that the frequencies of the X-ray lines do not jump about from element to element as atomic weights do. Their square roots go up in equal steps. If you number the elements in order, the square root of the frequency is a straight-line function of that number. Bohr had just published a theory of the hydrogen atom, and Moseley saw that the same theory, applied to an electron close to a nucleus of charge Z, explains his straight line almost exactly.

The empirical law

Moseley defined, for each element, a quantity

where \(\nu_0=R_\infty c\) is the Rydberg frequency, the frequency of the hydrogen ionization limit. He found that \(Q_K\) increases by almost exactly one from each element to the next. Calcium gave about 19, zinc about 29.

The factors \(\tfrac34=1-\tfrac14\) and \(\tfrac{5}{36}=\tfrac14-\tfrac19\) are the factors of the first lines of the Lyman and Balmer series of hydrogen. That is the clue to the explanation.

Derivation from the Bohr model

In Bohr’s 1913 theory an electron bound to a nucleus of charge \(Ze\) has energies \(E_n=-Z^2R_\infty hc/n^2\). In a many-electron atom an inner electron sees the nuclear charge reduced by the other electrons between it and the nucleus. Write the effective charge as \((Z-\sigma)e\), with \(\sigma\) a screening constant.

The Kα line is emitted when an electron drops from the \(n=2\) shell into a vacancy in the \(n=1\) shell. Before the jump, the K shell holds one remaining electron. An L electron therefore sees the nucleus shielded by about one charge, and in the crudest model one uses the same \(\sigma\approx1\) for both levels. Then

Dividing by \(h\) gives Moseley’s K law with \(b=1\). The same reasoning for the Lα line (\(n=3\to2\)) gives the factor \(\tfrac14-\tfrac19=\tfrac{5}{36}\); the L electrons are shielded by the two K electrons and by other L electrons, so \(b\) is larger, about 7.4. The vacancy picture was made explicit by Walther Kossel in 1914.

Moseley himself argued more cautiously. He noted that his results fitted the Bohr model if the K electrons formed a ring, and he stressed the fact that mattered most: whatever the mechanism, the integer \(Z\) that controls the X-ray spectrum is the same integer that orders the elements.

Atomic number and the periodic table

For the general reader. Dmitri Mendeleev ordered the elements by atomic weight, but he had to break the order in a few places to keep chemically similar elements together, and nobody could say how many elements were still undiscovered. Moseley’s law gave each element a number that could be measured. It confirmed Mendeleev’s exceptions, showed that there were exactly three gaps between aluminium and gold, and put an end to decades of confusion among the rare earths.

From position to charge

Rutherford’s scattering experiments of 1911 had shown that the atom has a small positive nucleus with charge roughly half the atomic weight in units of \(e\). In 1913 the Dutch amateur physicist Antonius van den Broek proposed that the nuclear charge equals the element’s serial position in the periodic table. Moseley’s data, he wrote in 1914, gave this hypothesis the strongest possible support. The integer \(Z\) in his law rises by one from element to element, and the Bohr derivation identifies it with the nuclear charge.

The Part II paper, published in April 1914, extended the K-series measurements from aluminium to silver and the L series from zirconium to gold, about thirty more elements.

The rare earths

The rare earths are chemically so alike that chemists could not agree how many there were. Moseley’s method answered the question at once, because each one gives its own lines regardless of chemistry. In 1914 Georges Urbain, the leading French rare-earth chemist, brought samples from Paris to Oxford to be examined by Moseley’s method. The X-ray picture fixed the lanthanides as the fifteen elements from lanthanum (57) to lutetium (71). That count left one more place open below tantalum: element 72.

Filling the gaps

Hafnium (72) was found in 1923 by Dirk Coster and George de Hevesy in Copenhagen, who identified its L lines in zirconium ores. Rhenium (75) was found in 1925 by Walter Noddack, Ida Tacke and Otto Berg. Technetium (43) was made artificially in 1937 by Carlo Perrier and Emilio Segrè from molybdenum irradiated in a cyclotron, and promethium (61) was identified in 1945 among fission products. Both have no stable isotopes, which is why chemists could not find them in nature.

Inner shells and screening

For the general reader. Moseley’s straight lines were not quite straight, and the small deviations turned out to be informative. They showed that the electrons in an atom are arranged in shells, that inner electrons partly hide the nucleus from outer ones, and that the innermost electrons of heavy atoms move fast enough for relativity to matter. X-ray spectroscopy became the main experimental source for the shell structure of atoms in the decade after Moseley.

The vacancy picture

Kossel’s interpretation of 1914 is the one used today. A fast electron or photon ejects an electron from an inner shell. An electron from a higher shell drops into the vacancy and emits a photon equal to the difference of the two binding energies. Kα comes from L → K, Kβ from M → K, Lα from M → L. The ejection requires at least the binding energy of the inner shell, so the absorption of X-rays by an element rises abruptly at that energy: the K and L absorption edges.

The model is rough. The screening constant for the K level is small, about 1 to 3 depending on \(Z\); for outer shells it grows. But it captures the main fact: X-ray energies scale nearly as \(Z^2\), while optical energies, involving outer electrons that see an almost fully screened nucleus, stay at a few electron-volts for all elements. This is why X-ray spectra are simple and regular while optical spectra are not.

X-ray spectroscopy after Moseley

For the general reader. Moseley’s method of telling elements apart by their X-ray lines is now used every day, in geology, metallurgy, archaeology, art conservation and on Mars. A beam of electrons or X-rays makes a sample glow with its characteristic lines, and the lines name every element present without any chemistry. The first triumph of the method, after his death, was the discovery of hafnium hidden in zirconium ores.

Precision spectroscopy

Manne Siegbahn in Lund refined the crystal spectrometer through the 1910s and 1920s, measuring X-ray wavelengths to parts in \(10^4\) and more. He mapped the K, L and M series of nearly all elements and received the Nobel Prize in Physics for 1924. Barkla had received the 1917 prize for the discovery of characteristic X-rays. The level scheme that emerged from this work, with its subshells and doublets, was one of the experimental supports for the electron spin and the exclusion principle in 1925.

Hafnium

After Bohr’s 1921–22 theory of the periodic table, element 72 was expected to be a chemical relative of zirconium, not a rare earth. Coster and de Hevesy, working at Bohr’s institute in Copenhagen, examined zirconium minerals with an X-ray spectrometer and found the L lines of element 72 at the positions Moseley’s law predicted. They announced it in January 1923 and named it hafnium, from the Latin name of Copenhagen.

Raimond Castaing’s electron microprobe of 1951 focused an electron beam to about a micrometre and analysed the X-rays from the spot, so that the composition of single mineral grains could be mapped. Portable X-ray fluorescence instruments now analyse paintings, alloys and soils in the field, and the PIXL instrument on NASA’s Perseverance rover maps elements in Martian rocks the same way.

Oxford, Australia and Gallipoli, 1913–1915

Oxford

Late in 1913 Moseley returned to Oxford, where he lived with his mother and worked in the laboratory of John Townsend, the professor of experimental physics. He had no post there. In the first months of 1914 he extended his measurements to the heavy elements and the L series and wrote Part II of his paper, published in April. He resigned his Manchester connection that spring.

Australia

In the summer of 1914 Moseley sailed to Australia for the meeting of the British Association for the Advancement of Science. War broke out in August, while the party was there. In Melbourne he took part in a discussion on the structure of the atom, and in Sydney he lectured on the X-ray spectra of the rare earths. He then returned to England as quickly as he could.

The army

Family and friends urged him not to enlist. Moseley regarded service as his duty and insisted on joining the fighting forces. After some difficulty he obtained a commission as a second lieutenant in the Royal Engineers and in October 1914 went to Aldershot for training as a signals officer. He was made signalling officer of the 38th Brigade in the 13th Division.

The division sailed in June 1915, by way of Alexandria, for the Gallipoli peninsula, where Allied forces had been pinned down since the landings in April. It took part in the August offensive around Anzac and Suvla Bay. On the morning of 10 August 1915, during the Ottoman counter-attack on the heights of Chunuk Bair, Moseley was shot in the head while telephoning an order to his division. He was 27. His body was never identified, and he is commemorated on the Helles Memorial.

Honours and legacy

Moseley received few honours in his lifetime; there was not time. Svante Arrhenius nominated him for the 1915 Nobel Prize in Chemistry, which went to Richard Willstätter. His death that August closed any later chance. The Italian Society of Sciences awarded him the Matteucci Medal in 1919. He left his estate to the Royal Society to support research, and a Royal Society studentship was founded from the bequest; its second holder was Patrick Blackett. The Institute of Physics awards the Henry Moseley Medal and Prize, and there are memorial plaques at Manchester and Eton.

Rutherford’s obituary in Nature, published in September 1915, set out his work in detail and made plain how much physics had lost. Bohr later recalled that the decisive change in how the nuclear atom was received came with Moseley’s X-ray work. Robert Millikan ranked his research among the most brilliant in the history of science.

Legacy

• Atomic number: Moseley made \(Z\) a measured physical quantity, the charge of the nucleus, and gave the periodic table its modern ordering principle.
• The nuclear atom and the Bohr model: his spectra were among the earliest and strongest evidence for both, coming from inner electrons where Bohr’s hydrogen theory applies best.
• The missing elements: his count of gaps guided the search for elements 43, 61, 72 and 75, all found by 1945.
• Analysis: X-ray fluorescence and electron microprobe analysis, the routine tools of materials science and geochemistry, are applications of his law.
• Science policy: his death became a standard example in arguments for protecting scientists in wartime, and the British approach to scientific manpower was different in the Second World War.

Connections

People linked to Henry Moseley across this collection. A name without a link has no page here yet. See Henry Moseley in the network of all 337 lives →

Teachers and advisors

• Ernest Rutherford 1910-1913

Influenced by

• Dmitri Mendeleev
• Lawrence Bragg
• Max von Laue
• Antonius van den Broek
• Charles Glover Barkla
• William Henry Bragg

Influenced

• Dirk Coster
• George de Hevesy
• Manne Siegbahn
• Walther Kossel

Colleagues and collaborators

• Hans Geiger 1910-1912
• Lawrence Bragg
• Niels Bohr 1912-1914
• Charles Galton Darwin 1912-1913
• Georges Urbain 1914
• John Sealy Townsend 1913-1914
• Kazimierz Fajans 1911

Family

• Henry Nottidge Moseley

Also mentioned in this life

• Svante Arrhenius
• Amedeo Avogadro
• J. J. Thomson
• Robert Millikan
• Arnold Sommerfeld

Fields

• atomic physics 7 lives
• radioactivity 10 lives

On CoursesHub

• Atomic & Optical Physics
• Nuclear Physics

Sources and further reading

Biography

1. Henry Moseley, Wikipedia (with extensive references).
2. E. Rutherford, Henry Gwyn Jeffreys Moseley (obituary), Nature 96 (1915), 33–34.
3. University of Manchester, Roll of Honour: Henry Gwyn Jeffreys Moseley.
4. University of Kansas Medical Center, Henry G. J. Moseley, WWI biography essay.
5. C. G. Darwin, Moseley’s determination of atomic numbers, recollection, IUCr.
6. J. L. Heilbron, H. G. J. Moseley: The Life and Letters of an English Physicist, 1887–1915, University of California Press, 1974.
7. Museum of the History of Science, Oxford: Moseley’s apparatus, crystals and plate holder.

Physics

1. H. G. J. Moseley, K. Fajans, Radio-active products of short life, Phil. Mag. (1911).
2. H. G. J. Moseley, The attainment of high potentials by the use of radium, Proc. R. Soc. A 88 (1913), 471–476.
3. H. G. J. Moseley, C. G. Darwin, The reflexion of X-rays, Phil. Mag. 26 (1913), 210.
4. H. G. J. Moseley, The high-frequency spectra of the elements, Phil. Mag. 26 (1913), 1024–1034; Part II, Phil. Mag. 27 (1914), 703–713 (annotated text, Le Moyne College classic papers).
5. W. L. Bragg, The diffraction of short electromagnetic waves by a crystal, Proc. Camb. Phil. Soc. 17 (1913).
6. A. van den Broek, Die Radioelemente, das periodische System und die Konstitution der Atome, Phys. Z. 14 (1913); and letters in Nature 92 (1913).
7. N. Bohr, On the constitution of atoms and molecules, Phil. Mag. 26 (1913).
8. D. Coster, G. von Hevesy, On the missing element of atomic number 72, Nature 111 (1923), 79.
9. M. Siegbahn, The Spectroscopy of X-rays, Oxford University Press, 1925.
10. J. Als-Nielsen, D. McMorrow, Elements of Modern X-ray Physics, 2nd ed., Wiley, 2011 (Bragg diffraction, X-ray sources).
11. R. Eisberg, R. Resnick, Quantum Physics of Atoms, Molecules, Solids, Nuclei and Particles, 2nd ed., Wiley, 1985, ch. 9 (X-ray spectra and Moseley’s law).