"It is extremely difficult to reconstruct the process by which Mendeleev came to his periodic organization of elements in terms of their atomic weights," Gordin wrote of the full periodic table. This is when he noticed certain types of elements regularly appearing and noticed a correlation between atomic weight and chemical properties.īut the exact Eureka! moment that led Mendeleev to the sorting strategy that produced his complete periodic table is shrouded in mystery. So according to the Royal Society of Chemistry, Mendeleev wrote the properties of each element on cards, and then he started ordering them by increasing atomic weight. But they weren't enough to usefully sort the 55 additional chemical elements known at the time. Gordin in his book "A Well-Ordered Thing: Dmitrii Mendeleev and the Shadow of the Periodic Table" (Princeton University Press, Revised Edition 2018). The first section of Mendeleev's book dealt with just eight of the known elements - carbon, hydrogen, oxygen, nitrogen, chlorine, fluorine, bromine and iodine - and those two strategies worked for those particular elements, according to Michael D. Just two strategies existed at the time to categorize these elements: separating them into metals and nonmetals or grouping them by an element's number of valence electrons (or those electrons in the outermost shell). At the time, there were 63 known chemical elements, each with an atomic weight calculated using Avogadro's hypothesis, which states that equal volumes of gases, when kept at the same temperature and pressure, hold the same number of molecules. Putting the elements in any kind of order would prove quite difficult. (Image credit: Oxford Science Archive/Print Collector/Getty Images)
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