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The periodic table arranges the elements of chemistry in seven horizontal rows called periods and 18 vertical columns called groups. It shows how chemical properties increase or decrease as you move down a group, and right to left across a period. The elements are also grouped according to their atomic number, which is equal to the number of protons found in each element’s nucleus.
Dmitry Mendeleev, a Russian chemist, created the periodic table in the mid-19th century. He arranged the elements into patterns that match their physical and chemical properties, using atomic number to guide his decisions. He even predicted that new elements would exist that hadn’t been discovered yet.
Henry Moseley, an English physicist, improved the periodic table. He measured the wavelengths of characteristic X-ray spectral lines for many elements and found that their frequencies depended on their atomic numbers—the same as the order in which they appear in the periodic table! This discovery, along with the earlier discoveries of noble gases helium, neon, argon, and krypton by Lord Rayleigh and William Ramsay, led to a compression of the periodic table into a “short-period” form that remains popular today.
Americium, the fourth synthetic transuranic element, was made by Glenn Seaborg, Ralph James, Leon Morgan, and Albert Ghiorso late in 1944 at the University of Chicago’s wartime Metallurgical Laboratory (now known as Argonne National Laboratory). It is a radioactive element with 19 isotopes and 8 nuclear isomers.
Claim reports for the discovery of new elements occur frequently in scientific literature. IUPAC and its member organizations assess these claims and recognize those laboratories whose research fulfills agreed criteria for a new element.