November 23 is Henry Moseley’s birthday. Moseley was the English physicist who gave meaning to the atomic number of an element.
Before Moseley’s work, the atomic number was just a placeholder to signify an element’s position on the periodic table. The elements were arranged mostly by atomic weight and their chemical properties. Moseley’s work related the atomic number to the charge contained in the element’s nucleus. This presented a physical property to order the elements. Eventually, it would be found this charge was due to particles called protons. Today, we determine an element by the number of protons in its nucleus.
Moseley began his physics career at an interesting time. His teacher, Ernest Rutherford had just confirmed the existence of the nucleus. Niels Bohr expanded this to show spectral lines of an element could be explained by transitions of energy levels of electrons. The father-son team of William and Lawrence Bragg were scattering electrons and producing x-rays.
Moseley brought all three of these together. He flooded samples of multiple elements with x-rays and recorded the spectrum produced by the interactions between the x-rays and the element’s electrons. He calculated the wavelengths of the x-rays emitted by the irradiated samples using Bragg’s equations. He found the energy of the emitted x-rays depended on the charge of atom’s nucleus. This mathematical relationship between energy and charge is known as Moseley’s Law. He also used this information to predict the existence of element 43, technetium and element 61, promethium which were holes in the periodic table.
When World War I broke out, Moseley left his research position to join the Army. He served as a Royal Engineer when the British invaded Turkey at Gallipoli. A Turkish sniper shot and killed him during the Battle of Gallipoli. He was only 27 years old.
Some have speculated he would have received the 1916 Nobel Prize had he lived. Many believed he would have ended up making significant contributions to physics. Enough people felt this way that the British government changed the rules of enlistment. Promising or prominent young scientists would be exempt from serving in combat duty.
Notable Science History Events for November 23
1887 – Henry Gwyn Jeffreys Moseley was born.
1865 – Friedrich Georg Wilhelm von Struve died.
Struve was a German astronomer who was a pioneer in the study of binary stars. He was the second in a five generation series of Struve astronomers. Struve was also one of the first astronomers to measure the parallax of a star.
Astronomers had noted several stars appeared to work in pairs close to each other. As telescopes improved, many of these double stars turned out to be optical illusions. Struve made precise observations of double stars and catalogued 2714 true binary star systems of two stars orbiting each other.
Struve also set up the Struve Geodetic Arc. This project was supposed to generate an accurate measurement of the flattening of the Earth due to its rotation and a measurement of the equator’s radius. A chain of surveying markers was constructed ranging from Hammerfest, Norway to Nekrasivika, Odessa Oblast in Ukraine. The distance between these two points was measured to 2,821.853 km with an error of only ± 12 meters. Struve’s published value for the flattening of the Earth as one part in 294.26 and the equator’s radius at 6,378,360.7 meters. Current satellite data shows a flattening of one part in 298.257 and a 6,378,136.8 meters.
The Struve Geodetic Arc measurement sites are currently part of the UNESCO World Heritage List with markers at the original 256 measuring stations.
1837 – Johannes Diderik van der Waals was born.
Johannes Diderik van der Waals was a Dutch physical chemist who was awarded the 1910 Nobel Prize in Physics for describing a state of matter where the liquid and gas phases merge together continuously. He was the first to postulate an intermolecular force. He also derived a general equation for the ideal gas equation taking into account the attractive forces and volumes of the molecules.
1826 – Johann Elert Bode died.
Bode was a German astronomer who determined the orbit of Uranus. He is also known for his empirical law to determine the distances of the planets from the Sun. To use Bode’s law, start with the sequence 0, 3, 6, 12, 24, etc. where each number after 3 is twice the previous number. Add 4 to each number and divide the result by 10. The resulting first six numbers of the sequence are 0.4, 0.7, 1.0, 1.6, 2.8, 5.2, and 10.0. These values closely approximate the distances in astronomical units of the planets from the Sun.
1221 – Alfonso X was born.
Alphonso X of Castile was a Spanish monarch and astronomer who published planetary tables to chart positions of the planets. The Alphonsine tables served as the standard tables for over 300 years during the Middle Ages.