Follow the grand historical axis through Paris from the Arc de Triomphe, along the Champs Élysées, through the Place de la Concorde and the Tuilleries Gardens and you will arrive at the spectacular glass Louvre Pyramid. This crystalline entrance to the Louvre Museum, modelled on the pyramids of Ancient Egypt, unites the antique and the modern in a single elegant structure.
The Louvre Museum
Hidden fifteen metres underground directly beneath the pyramid is a particle accelerator with the ability to reveal the secrets of the treasures of antiquity. This machine is known as AGLAE (Accélérateur Grand Louvre d’analyse élémentaire).
The Seated Scribe under analysis from AGLAE.
To understand what is going on in the heart of Paris we must travel back in time just over a century to the bustling industrial city of Manchester.
The Rutherford Planetary Atom
Between March and July 1912 the young Danish theorist Niels Bohr (1885-1962) worked with Ernest Rutherford’s team at the University of Manchester. Rutherford’s model of the atom had been published the previous year and his team were busily studying its structure. During these critical months Bohr realised that the new quantum ideas of Planck and Einstein might provide a foundation for understanding the atom.
Albert Square, Manchester by Adolphe Valette (1910).
In Rutherford’s scheme atoms were formed of a tiny positively charged nucleus orbited by negatively charged electrons. The nucleus of the simplest atom—hydrogen—carried a single positive electric charge and was orbited by a single electron. Bohr devised a mathematical model of the atom in which electrons could only occupy a fixed series of energy levels. They could be boosted to a higher energy level by absorbing a photon of light with just the right energy. They could also drop down to a lower energy level emitting a photon to take away the energy difference between the two levels.
Bohr’s ideas offered a qualitative understanding of spectroscopy, a technique that enabled astronomers to determine the chemical make-up of the stars by matching the dark spectral lines seen in light from the stars to the bright spectral emission lines observed in the laboratory. Even more remarkable was the numerical agreement between the wavelengths of the lines in the hydrogen spectrum and the predictions of Bohr’s model of the hydrogen atom. This precise correspondence demonstrated the fundamental validity of Bohr’s model and Bohr was rewarded with the 1922 Nobel Prize in Physics.
Bohr’s groundbreaking work was soon followed by another application of his model that is less well known. It was made by Henry Moseley who joined Rutherford’s team in Manchester after graduating from Oxford in 1910. Moseley’s contribution would result in a major step forward in understanding the structure of atoms and in explaining the Periodic Table of the Elements.
X-ray Spectra
When Russian chemist Dmitri Mendeleyev (1834–1907) devised the Periodic Table he arranged the elements in order of increasing atomic mass, and this was still how the table was organised in the early years of the twentieth century.
The Periodic Table (1871)
But in 1913 a Dutch physicist Anton van den Broek (1870-1926) suggested that perhaps the elements should be ordered by nuclear charge rather than atomic mass. Moseley took up the challenge of testing whether this was true. Bombarding a pure sample of an element causes it to emit X-rays of a characteristic wavelength. This is just like the visible light spectrum of an element, but with much higher energy electromagnetic radiation. Moseley assumed correctly that this happens when one of the innermost electrons is knocked out of the atom leaving a vacancy in an inner orbital. This vacancy is then filled by an electron from a higher energy orbital with the emission of an X-ray photon that carries away the energy difference between the two orbitals.
Moseley systematically bombarded pure samples of each element with an electron beam and determined the wavelength of the emitted X-rays by measuring their angle of deflection when passed through a crystal. He interpreted these results by adapting Bohr’s formula for the electron energy levels to give a relationship between the nuclear charge of an atom and the energy of the emitted X-rays. This enabled him to deduce the nuclear charge of each element. These results were summarised in the first part of a two-part paper where Moseley wrote:
We have here a proof that there is in the atom a fundamental quantity, which increases by regular steps as we pass from one element to the next. The quantity can only be the charge on the central positive nucleus
But by November 1913 the Manchester weather and the thick yellow smog had become too much for Moseley. He returned to Oxford and continued his work there.
Henry Moseley in his laboratory in Oxford.
Moseley analysed the X-ray spectra of each element from aluminium to gold, and by the following April he had submitted the second part of his paper. His brilliant experimental work had shown that the nucleus of an atom carries a charge that is a multiple of the charge on the hydrogen nucleus. This multiple is known as the atomic number. And he had conclusively demonstrated that the elements should be ordered by atomic number not atomic mass.
Chemical Anomalies
Moseley noted that this resolved three anomalies in the Periodic Table where the positions of two elements were switched to match their chemical properties. Ever since Mendeleyev, chemists had swapped the positions of iodine and tellurium because, even although iodine has a lower atomic mass than tellurium, it was clear that it belongs in Group VII with the halogens, whereas tellurium belongs in Group VI beneath selenium. The need for this sleight of hand was now removed as Moseley showed that iodine has atomic number 53, whereas tellurium has atomic number 52. Moseley accounted for similar switches in the cases of argon and potassium, and cobalt and nickel.
Moseley also concluded that three elements were missing between element-13 aluminium and element-79 gold. These elements would have atomic numbers 43, 61 and 75. In subsequent decades each would be tracked down. They are known as technetium, promethium and rhenium.
The Element Celtium
Moseley also suggested that the element celtium, announced by Georges Urbain in 1911, required further investigation. Moseley suspected that celtium might be element-72. But you won’t find celtium in the Periodic Table today. Following the publication of Moseley’s paper, Urbain visited Oxford and Moseley tested a sample of his celtium. There was no sign of any lines in the X-ray spectrum corresponding to atomic number 72. Urbain returned to France disappointed, but amazed that Moseley was able to complete his analysis in a single day. Element-72 was eventually isolated in 1923. It is known as hafnium.
The modern Periodic Table of the Elements
The Proton
Moseley’s results almost screamed out that the nucleus contains a particle with opposite but equal charge to the electron. At least this conclusion seems obvious in hindsight. Three years on, in 1917, Rutherford blasted nitrogen with alpha particles and liberated positively charged particles from the nitrogen nuclei. He named them protons.
The discovery of the proton represented a major leap forward. Atomic number is simply the number of protons in the nucleus. It was now clear how this number determines the atom’s chemical properties. Each proton carries one unit of positive electric charge. In a neutral atom this charge is balanced by an equal number of orbiting electrons, each carrying one unit of negative electric charge. Electrons are responsible for chemically bonding atoms together into molecules and solids, but the electric charge of the nucleus determines the number of electrons available to form these bonds.
The Great War
In August 1914 war broke out and Moseley volunteered for the Royal Engineers, leaving for the Dardanelles in June 1915. Two months later he was killed by a sniper’s bullet during the Gallipoli campaign. He was 27. There is little doubt that Moseley deserved a Nobel Prize for his work. But Nobel Prizes are never awarded posthumously.
The magnitude of Moseley’s discoveries can be judged from a surprising comment Bohr made to an interviewer in 1962. According to Bohr, prior to Moseley’s work no one took Rutherford’s planetary atom seriously:
There was no mention of it in any place. The great change came from Moseley.
Votive crown of the Visigoth King Recceswinth. Part of the Guarrazar Treasure.
Hidden Treasures
Beneath the pyramid outside the Louvre precious artefacts are interrogated with proton beams using a technique known as PIXE (Particle Induced X-ray Emission) to reveal their atomic constituents and identify their origins. The Louvre scientists are following in Moseley’s footsteps, using his methods to investigate the chemical composition of artworks such as stained glass from the windows of Chartres Cathedral, a solid gold scabbard donated to Napoleon Bonaparte by the French Republic and the eyes of a sculpted Ancient Egyptian scribe. AGLAE has also analysed the Guarrazar treasure discovered near Toledo in Spain in 1859 and dating back to the seventh century Visigoths. This spectacular treasure contains golden crowns and crosses decorated with pearls, emeralds, garnets and sapphires from locations as distant as Sri Lanka.
Further Information
There is more about AGLAE here: The accelerator in the Louvre.
