Henry Moseley

Henry G. J. Moseley, known to his friends as Harry, was born in Weymouth in Dorset in 1887. His father Henry Nottidge Moseley (1844–91), who died when Henry Moseley was quite young, was a Biologist and also a professor of anatomy and physiology at the University of Oxford, who had been a member of the Challenger Expedition. Moseley's mother was Amabel Gwyn Jeffreys Moseley, the daughter of the Welsh Biologist and conchologist John Gwyn Jeffreys.

Henry Moseley had been a very promising schoolboy at Summer Fields School (where one of the four 'leagues' is named after him), and he was awarded a King's scholarship to attend Eton College. In 1906 he won the chemistry and physics prizes at Eton. In 1906, Moseley entered Trinity College of the University of Oxford, where he earned his bachelor's degree. Immediately after graduation from Oxford in 1910, Moseley became a demonstrator in physics at the University of Manchester under the supervision of Sir Ernest Rutherford. During Moseley's first year at Manchester, he had a teaching load as a graduate teaching assistant, but following that first year, he was reassigned from his teaching duties to work as a graduate research assistant. He declined a fellowship offered by Rutherford, preferring to move back to Oxford, in November 1913, where he was given laboratory facilities but no support.

Before Moseley and his law, atomic numbers had been thought of as a semi-arbitrary ordering number, vaguely increasing with atomic weight but not strictly defined by it. Moseley's discovery showed that atomic numbers were not arbitrarily assigned, but rather, they have a definite physical basis. Moseley postulated that each successive element has a nuclear charge exactly one unit greater than its predecessor. Moseley redefined the idea of atomic numbers from its previous status as an ad hoc numerical tag to help sorting the elements into an exact sequence of ascending atomic numbers that made the Periodic Table exact. (This was later to be the basis of the Aufbau principle in atomic studies.) As noted by Bohr, Moseley's law provided a reasonably complete experimental set of data that supported the (new from 1911) conception by Ernest Rutherford and Antonius van den Broek of the atom, with a positively charged nucleus surrounded by negatively charged electrons in which the atomic number is understood to be the exact physical number of positive charges (later discovered and called protons) in the central atomic nuclei of the elements. Moseley mentioned the two Scientists above in his research paper, but he did not actually mention Bohr, who was rather new on the scene then. Simple modification of Rydberg's and Bohr's formulas were found to give theoretical justification for Moseley's empirically derived law for determining atomic numbers.

Experimenting with the Energy of beta particles in 1912, Moseley showed that high potentials were attainable from a radioactive source of radium, thereby inventing the first atomic battery, though he was unable to produce the 1MeV necessary to stop the particles.

In 1913, Moseley observed and measured the X-ray spectra of various chemical elements (mostly metals) that were found by the method of diffraction through crystals. This was a pioneering use of the method of X-ray spectroscopy in physics, using Bragg's diffraction law to determine the X-ray wavelengths. Moseley discovered a systematic mathematical relationship between the wavelengths of the X-rays produced and the atomic numbers of the metals that were used as the targets in X-ray tubes. This has become known as Moseley's law.

Isaac Asimov also speculated that, in the event that he had not been killed while in the Service of the British Empire, Moseley might very well have been awarded the 1916 Nobel Prize in Physics, which, along with the prize for chemistry, was not awarded to anyone that year. Additional credence is given to this idea by noting the recipients of the Nobel Prize in Physics in the two preceding years, 1914 and 1915, and in the following year, 1917. In 1914, Max Von Laue of Germany won the Nobel Prize in Physics for his discovery of the diffraction of X-rays by crystals, which was a crucial step towards the invention of X-ray spectroscopy. Then, in 1915, William Henry Bragg and william Lawrence Bragg, a British father-son pair, shared this Nobel Prize for their discoveries in the reverse Problem — determining the structure of crystals using X-rays (Robert Charles Bragg, william Henry Bragg's other son, had also been killed at Gallipoli, on 2 September 1915). Next, Moseley used the diffraction of X-rays by known crystals in measuring the X-ray spectra of metals. This was the first use of X-ray spectroscopy and also one more step towards the creation of X-ray crystallography. In addition, Moseley's methods and analyses substantially supported the concept of atomic number, placing it on a firm, physics-based foundation. Moreover, Charles Barkla of Great Britain was awarded the Nobel Prize in 1917 for his experimental work in using X-ray spectroscopy in discovering the characteristic X-ray frequencies emitted by the various elements, especially the metals. "Siegbahn, who carried on Moseley's work, received one [the 1924 Nobel Prize in Physics]." Moseley's discoveries were thus of the same scope as those of his peers, and in addition, Moseley made the larger step of demonstrating the actual foundation of atomic numbers. Ernest Rutherford commented that Moseley's work, "Allowed him to complete during two years at the outset of his career a set of researches that would surely have brought him a Nobel prize".

In addition, Moseley showed that there were gaps in the atomic number sequence at numbers 43, 61, 72, and 75. These spaces are now known, respectively, to be the places of the radioactive synthetic elements technetium and promethium, and also the last two quite rare naturally occurring stable elements hafnium (discovered 1923) and rhenium (discovered 1925). Nothing was known about these four elements in Moseley's lifetime, not even their very existence. Based on the intuition of a very experienced Chemist, Dmitri Mendeleev had predicted the existence of a missing element in the Periodic Table, which was later found to be filled by technetium, and Bohuslav Brauner had predicted the existence of another missing element in this Table, which was later found to be filled by promethium. Henry Moseley's experiments confirmed these predictions, by showing exactly what the missing atomic numbers were, 43 and 61. In addition, Moseley predicted the existence of two more undiscovered elements, those with the atomic numbers 72 and 75, and gave very strong evidence that there were no other gaps in the Periodic Table between the elements aluminium (atomic number 13) and gold (atomic number 79).

Only twenty-seven years old at the time of his death, Moseley could, in the opinion of some Scientists, have contributed much to the knowledge of atomic structure had he survived. Niels Bohr said in 1962 that the Rutherford's work "was not taken seriously at all" and that the "great change came from Moseley."

This latter question about the possibility of more undiscovered ("missing") elements had been a standing Problem among the chemists of the world, particularly given the existence of the large family of the lanthanide series of rare earth elements. Moseley was able to demonstrate that these lanthanide elements, i.e. lanthanum through lutetium, must have exactly 15 members – no more and no less. The number of elements in the lanthanides had been a question that was very far from being settled by the chemists of the early 20th Century. They could not yet produce pure samples of all the rare-earth elements, even in the form of their salts, and in some cases they were unable to distinguish between mixtures of two very similar (adjacent) rare-earth elements from the nearby pure metals in the Periodic Table. For Example, there was a so-called "element" that was even given the chemical name of "didymium". "Didymium" was found some years later to be simply a mixture of two genuine rare-earth elements, and these were given the names neodymium and praseodymium, meaning "new twin" and "green twin". Also, the method of separating the rare-earth elements by the method of ion exchange had not been invented yet in Moseley's time.