Today, we take the concept of atomic number for granted. Atomic number is the measure of the number of positively charged protons within the nucleus of an atom, and it defines what an element is.
For example, the element oxygen, which has atomic number 8, is very different from the element lead, which has atomic number 82 or the element iodine, which has atomic number 53. The man who first elucidated the concept of atomic number is the British physicist Henry Moseley.
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A brilliant start
Henry Moseley was born on November 23, 1887 in Weymouth, England to a naturalist father, Henry Nottidge Moseley, who had been a member of the Challenger Expedition. The members of this expedition traveled over 81,000 miles (130,000 km) around the globe, surveying and exploring the world's oceans.
Henry Moseley's mother was the daughter of Welsh biologist John Gwyn Jeffreys, and was herself a British chess champion. In a case of the apple not falling far from the tree, Henry Moseley excelled at chemistry and physics, first at Eton College, and then at Trinity College, Oxford.
In 1910, Moseley moved to the University of Manchester to join Ernest Rutherford's research group and to teach. Rutherford, who is known as the father of nuclear physics, is the discoverer of the half-life of radioactive elements, discoverer of the element radon. And he differentiated alpha radiation from beta radiation.
At Manchester, Moseley created the world's first atomic battery, or beta-cell. Today, atomic batteries are used anywhere power is needed for a long time, such as in cardiac pacemakers and in spacecrafts.
Taming the periodic table of the elements
The periodic table of the elements had been created by the Russian chemist Dimitri Mendeleev, 44 years earlier in 1869. Elements were arranged on it according to their atomic weight and their chemical properties. Then, in 1911, Dutch physicist Antonius van den Broek published a hypothesis that stated that there was something called the atomic number, and that it was equal to the amount of charge in an atom's nucleus.
In 1913, Moseley returned to Oxford where he had to self-fund his experiments. He set up apparatus that shot high-energy electrons at various chemical elements, and he then measured the wavelengths and frequencies of the resulting x-rays.
Moseley discovered that each element emits x-rays at a unique frequency, and he discovered that if he plotted the square-root of the x-ray's frequency against the various elements' atomic numbers, he got a straight line graph.
This data showed that the positive charge in an atomic nucleus increased by one unit from one element to the next in the periodic table. Thus, the atomic number is the same as the number of protons in the nucleus. This work became known as Moseley's Law.
Prior to Moseley's discovery, it had been difficult to order elements such as Cobalt and Nickel, which have the atomic numbers 27 and 28 respectively, because the atomic mass of Cobalt is actually slightly higher than that of Nickel.
Most importantly, Moseley saw that there were gaps in the periodic table at the atomic numbers: 43, 61, 72 and 75. It would take years before it was known that these numbers corresponded to the elements Technetium, Promethium, Hafnium and Rhenium.
Moseley could now tell what elements are present in any sample by bombarding that sample with high-energy electrons, then looking at the frequencies of the resulting x-rays. Called x-ray spectroscopy, today this technique is used in laboratories around the world.
Below is the results of x-ray spectroscopy that was performed by the Mars Pathfinder lander on samples of Martian soil.
Moseley was able to show that the lanthanide series of chemical elements is comprised of exactly 15 metallic chemical elements, having the atomic numbers 57 through 71. These numbers correspond to the elements Lanthanum to Lutetium.
Along with their sister elements, Scandium and Yttrium, these elements are known as the rare earth elements, and they are extremely useful in today's world. Rare earth elements are used in smartphones, digital cameras, computer hard disks, fluorescent and LED lights, flat screen televisions, computer monitors, and electronic displays.
World War I
In August 1914, World War I broke out, and Moseley enlisted in the British Army's Royal Engineers, feeling it was his patriotic duty.
From February 1915 to January 1916, in what is today Gelibolu, Turkey, Britain, France and Russia had been attempting to take control of the Dardanelles. This is the narrow strait of water that forms part of the boundary between Europe and Asia.
Moseley was serving as a technical communications officer at the Battle of Gallipoli when on August 10, 1915, he was shot in the head by a sniper. Moseley was only 27-years-old when he died, and he is buried on Turkey's Gallipoli Peninsula.
Moseley's place in history
Over the years, scientists such as Niels Bohr commented on how had Moseley lived, he would have contributed much to the knowledge of atomic structure. American physicist Robert Millikan wrote of Moseley's work:
"In a research which is destined to rank as one of the dozen most brilliant in conception, skillful in execution, and illuminating in results in the history of science, a young man twenty-six-years-old threw open the windows through which we can glimpse the sub-atomic world with a definiteness and certainty never dreamed of before."
Famous American science fiction author Isaac Asimov wrote of Moseley:
"In view of what he [Moseley] might still have accomplished … his death might well have been the most costly single death of the War to mankind generally."
Had he lived, Moseley almost assuredly would have been awarded a Nobel Prize in Physics because in 1914, that prize was awarded to German Max von Laue for his discovery of the diffraction of X-rays by crystals. In 1915, the Nobel Prize in Physics went to British father and son William Henry Bragg and Lawrence Bragg for their discoveries in determining the structure of crystals using x-rays.
No Nobel Prize in Chemistry or Physics was awarded in 1916, however, in 1917, Britain Charles Barkla received the prize for his work in discovering the characteristic x-ray frequencies emitted by the various elements.
Today, the Institute of Physics Henry Moseley Medal and Prize is named in Moseley's honor.