When contemplating those weights, Döbereiner noted that certain sets of three elements (he called them triads) showed a peculiar relationship. Measuring atomic weights accurately became a prime preoccupation for chemists in the decades that followed. 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.ĭalton 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. The periodic table symbolizes not merely the constituents of matter, but the logical cogency and principled rationality of all science.Ĭhemical reactions, Dalton proposed, produced new substances when atoms were disconnected or joined. 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. Mendeleev’s table did more than foretell the existence of new elements. “The law of periodicity first enabled us to perceive undiscovered elements at a distance which formerly was inaccessible to chemical vision.” “Before the promulgation of this law the chemical elements were mere fragmentary, incidental facts in Nature,” 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. Mendeleev’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. “The periodic table,” wrote the chemist Peter Atkins, “is arguably the most important concept in chemistry.”
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