But his theory made the idea of atoms useful, inspiring a revolution in chemistry. When elements combined to make new substances, the amounts that reacted could be calculated with knowledge of those atomic weights.Дalton was wrong about some of the weights - oxygen is really 16 times the weight of hydrogen, and carbon is 12 times heavier than hydrogen. Oxygen atoms weighed eight times as much as hydrogen atoms carbon atoms were six times as heavy as hydrogen, Dalton believed. Any given element consisted entirely of one kind of atom, he reasoned, distinguished from other kinds by weight. In his New System of Chemical Philosophy, Dalton explained chemical reactions by assuming that each elementary substance was made of a particular type of atom.Д¬hemical reactions, Dalton proposed, produced new substances when atoms were disconnected or joined. In those days, chemists hadnвЂ™t yet fully grasped the nature of atoms, as described in the atomic theory proposed by English schoolteacher John Dalton in 1808. In fact, German chemist Johann Wolfgang DГ¶bereiner noticed peculiarities in groupings of elements as early as 1817. Mendeleev had been thinking about grouping the elements for years, and other chemists had considered the notion of relationships among the elements several times in the preceding decades. Legend has it that Mendeleev conceived and created his table in a single day: February 17, 1869, on the Russian calendar (March 1 in most of the rest of the world). Historic Images/Alamy Stock Photo Laying the groundwork The periodic table symbolizes not merely the constituents of matter, but the logical cogency and principled rationality of all science.

His table finished the transformation of chemical science from the medieval magical mysticism of alchemy to the realm of modern scientific rigor. It hinted at the existence of subatomic structure and anticipated the mathematical apparatus underlying the rules governing matter that eventually revealed itself in quantum theory. It validated the then-controversial belief in the reality of atoms. MendeleevвЂ™s table did more than foretell the existence of new elements. вЂњThe law of periodicity first enabled us to perceive undiscovered elements at a distance which formerly was inaccessible to chemical vision.вЂќ вЂњBefore the promulgation of this law the chemical elements were mere fragmentary, incidental facts in Nature,вЂќ Mendeleev declared. His law revealed profound familial relationships among the known chemical elements - they exhibited similar properties at regular intervals (or periods) when arranged in order of their atomic weights - and enabled Mendeleev to predict the existence of elements that had not yet been discovered. MendeleevвЂ™s table looked like an ad hoc chart, but he intended the table to express a deep scientific truth he had uncovered: the periodic law. вЂњThe periodic table,вЂќ wrote the chemist Peter Atkins, вЂњis arguably the most important concept in chemistry.вЂќ

Mendeleev periodic table