Mostly Moseley

“It has been the fate of some men to accomplish in their youth a work of surpassing importance, and then to have their career suddenly cut short by a great catastrophe.” Thus writes Bernard Jaffe in his book Crucibles of Chemistry in his chapter on Henry Moseley. How do we distinguish the elements in the periodic table from one another? Students learn this early on in chemistry class: By atomic number – the number of protons that an atom of an element possesses. It was Moseley who figured this out.

 

Shortly after earning his undergraduate degree, Moseley joined the research lab of Ernest Rutherford (of gold-foil experiment fame). At that time, anyone who joined the lab “went through a period of intensive training and experimentation in electricity, magnetism, optics, and radioactivity.” Thus, Moseley was very well trained as an experimentalist. His first project to determine the half-life of an actinium isotope led to his first published paper in 1911. That same year, Geiger and Marsden working in Rutherford’s lab determined that the positive charge of the atomic nucleus was roughly half its relative atomic mass. Moseley’s task was to figure out how exactly charge and mass were correlated.

 

To do this, Moseley fired electrons at a metal plate. The metal emanated X-rays and the characteristic lines of these X-ray spectra were photographed. Putting together these spectra led to the generated pattern known as “Moseley’s staircase” (shown below). The work was not easy. Building a device that allowed him to make these measurements was painstaking, but Moseley had the ingenuity, skills, and most importantly patience, to overcome one problem after another. In 1912, after a solid six months of work, he published his “Law of Atomic Numbers”. It was integrated into Mendeleev’s periodic table, helping to confirm Mendeleev’s intuition but also correct some of the discrepant ordering. Moseley was greatly aided by his discussions with Bohr who went on to publish his famous model of the atom in 1913.

 

Moseley’s work also helped disprove many claims that yet another new element had been discovering: “More than seventy elements had been announced during the [previous] generation to fill sixteen gaps in Mendeleev’s Table.” Moseley whittled the seventy down to nine, leaving only seven remaining gaps. Moseley even made predictions of what their X-ray spectra would look like, and that they will soon be found. The story of the discovery of these seven is now well known. Moseley’s law of atomic numbers also revived Prout’s Hypothesis, proposed a century before, but without gaining traction. Prout proposed that hydrogen was the basis of everything, and that all other elements were multiples of hydrogen. Where nuclei of atoms are considered, that’s not far from the truth.

 

Moseley enthusiastically signed up for active duty and went to the front lines in World War I, despite the pleading of Rutherford that he should work at a military research lab as his service. Moseley was killed at Gallipoli. He was twenty-six years old. His colleagues and fellow officers described him as a fearless man full of good cheer. He might well have won the Nobel prize, or played a key role in finding isotopes, and we can only speculate on what else he would have accomplished scientifically. Jaffe suggests we call atomic numbers Moseley Numbers; I like that idea!