Henry Gwyn Jeffreys Moseley | Biography

Henry Gwyn Jeffreys Moseley's great contribution to science was his ordering of the elements by examining their X-ray spectra, thus creating the periodic table. His work, which ranks among the most profound discoveries of the twentieth century, was cut short when the scientist was killed during World War I. Although he never received a major award for his research, Moseley's accomplishments laid the groundwork for a number of later scientific developments.

Henry Gwyn Jeffreys Moseley (widely known as "Harry") was born on November 23, 1887, at Weymouth, England. He was the only son of Henry Nottidge Moseley, a professor of human and comparative anatomy at the University of Oxford. He had also served as a naturalist on the famous voyages of the Challenger in that vessel's studies of the world's oceans. Both sides of the family included a number of eminent scientists, including a paternal grandfather who was the first professor of natural philosophy at King's College, London, and a great grandfather who was an expert on tropical diseases.

Moseley's father died when Harry was four years old. The family then moved to the small town of Chilworth, in Surrey, where Harry and his two sisters began their education. Moseley's interest in science surfaced at a young age. He was fortunate in having the constant encouragement of his mother and friends in this interest which was to lead not only to a professional career in science, but also to a lifelong fascination with natural history.

In 1896, at the age of nine, Moseley was enrolled at the Summer Fields school, an institution that specialized in preparing boys for Eton. Five years later, he won a King's Scholarship that allowed him to enroll at Eton. At Eton, Moseley studied with T. C. Porter , one of the first scientists in England to work with X rays. Although Moseley studied a number of other subjects, X rays became the topic in which he was interested during his career in science.

After leaving Eton in 1906, Moseley was awarded a scholarship to continue his education at Trinity College, Oxford. Though he earned only second class honors in science, Moseley was able to get letters of recommendation that allowed him to take a position at the University of Manchester, where he worked with Ernest Rutherford. His assignment at Manchester involved a full teaching load, but he still found time to carry out an ambitious program of original research.

At the conclusion of his first year at Manchester, Moseley was relieved of his teaching responsibilities and allowed to devote all his time to research. The topic he selected for that research was the diffraction of X rays, a phenomenon that had just been discovered by the German physicist Max Laue. For a period of some months, Moseley collaborated with C. G. Darwin , a mathematical physicist, on a study of the general characteristics of diffracted X rays. In the fall of 1913, however, the two men went their separate ways, and Moseley began to focus on the nature of the spectra produced by scattered X rays.

Moseley saw that when X rays are beamed at certain crystalline materials, they are diffracted by atoms within the crystals, forming a continuous spectrum on which is superimposed a series of bright lines. The number and location of these lines are characteristic of the element or elements being studied. Much of the basic research on X-ray spectroscopy had been done in England by William Lawrence Bragg, with whom Moseley studied for a short period of time at the University of Leeds.

In his own research, Moseley devised a system that allowed him to study the X-ray diffraction pattern produced by one element after another in an orderly and efficient arrangement. Very quickly, he found that the frequencies of one set of spectral lines, the "K" lines, differed from element to element in a very consistent and orderly way. That is, when the elements were arranged in ascending order according to their atomic masses, the frequency of their K spectral lines differed from each other by a factor of one. To Moseley, the meaning of these results was clear. Some property inherent in the structure of atoms was responsible for the regular, integral change he observed. That property, he decided, was the charge on the nucleus. When the elements are arranged in ascending order to their atomic masses, he pointed out, they are also arranged in ascending order according to their nuclear charge. The main difference is that the variation in atomic masses between adjacent elements is never consistent, whereas the variation in nuclear charge is always precisely one. This property is such a clear defining characteristic of atoms and elements that it was given the special name of atomic number. Of all the properties of an atom, atomic number has come to be the single most important characteristic by which an atom can be recognized.

The implications of Moseley's discovery were manifold and profound. In the first place, the concept of a unique, identifying and characteristic number of an element--its nuclear charge--provided a new basis for the periodic table. The Russian chemist Dmitri Ivanovich Mendeleev 's original proposal for arranging the elements on the basis of their atomic masses had worked extraordinarily well, but it did have its flaws. Moseley found, however, that the discrepancies found in Mendeleev's proposal disappeared when the elements were arranged according to their atomic numbers. It was obvious, therefore, that atomic number was an even more fundamental property of atoms and elements than was atomic mass.

Moseley's research also allowed him to predict the number and location of elements still missing from the periodic table. Since each element could be assigned its own unique atomic number (hydrogen=1, helium=2, lithium=3, beryllium=4, and so on), any missing numbers must mean that an element remains to be discovered for this particular position in the periodic table. Based on that logic, Moseley was able to predict with confidence the existence of an as-yet-undiscovered element #43, between molybdenum (#42) and ruthenium (#44) and another between neodymium (#60) and samarium (#62). Furthermore, he was able to predict the spectral pattern to be expected for each missing element and thereby provide a valuable tool in the search for those elements.

As his initial work on X-ray spectra was being concluded, Moseley decided to resign his position at Manchester and return to Oxford in the hope of securing a position as professor of experimental physics. At Oxford, Moseley continued to work on X-ray spectra, concentrating on the study of the relatively unknown group of elements known as the lanthanides.

In June of 1914, Moseley left for a meeting of the British Association for the Advancement of Science scheduled to be held in Australia. He received word while still aboard ship of the outbreak of World War I and decided to return to England immediately. He enlisted in the Royal Engineers and, after completing an eight-month training period, was sent to the Turkish battlefront at Gallipoli. There, during the battle of Sulva Bay on June 15, 1915, he was killed by a sniper's bullet. Moseley was never married and, in his short lifetime, received no great honors. His contributions to science, however, were enormous, in that many later advances in physics and chemistry were based on his work.