2.1 Early Ideas in Atomic Theory - Chemistry 2e _ OpenStax.pdf

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7/3/20202.1 Early Ideas in Atomic Theory - Chemistry 2e | OpenStax1/6By the end of this section, you will be able to:State the postulates of Dalton’s atomic theoryUse postulates of Dalton’s atomic theory to explain the laws of definite and multiple proportionsThe earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers,the scientists of their day. In the fifth century BC, Leucippus and Democritus argued that all matter wascomposed of small, finite particles that they calledatomos, a term derived from the Greek word for“indivisible.” They thought of atoms as moving particles that differed in shape and size, and which could jointogether. Later, Aristotle and others came to the conclusion that matter consisted of various combinations ofthe four “elements”—fire, earth, air, and water—and could be infinitely divided. Interestingly, thesephilosophers thought about atoms and “elements” as philosophical concepts, but apparently neverconsidered performing experiments to test their ideas.The Aristotelian view of the composition of matter held sway for over two thousand years, until Englishschoolteacher John Dalton helped to revolutionize chemistry with his hypothesis that the behavior of mattercould be explained using an atomic theory. First published in 1807, many of Dalton’s hypotheses about themicroscopic features of matter are still valid in modern atomic theory. Here are the postulates ofDalton’satomic theory.1. Matter is composed of exceedingly small particles called atoms. An atom is the smallest unit of anelement that can participate in a chemical change.2. An element consists of only one type of atom, which has a mass that is characteristic of the elementand is the same for all atoms of that element (Figure 2.2). A macroscopic sample of an elementcontains an incredibly large number of atoms, all of which have identical chemical properties.Figure 2.2A pre-1982 copper penny (left) contains approximately 310copper atoms (several dozen are represented as brown spheres atthe right), each of which has the same chemical properties. (credit:modification of work by “slgckgc”/Flickr)3. Atoms of one element differ in properties from atoms of all other elements.

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Term
Summer
Professor
CHIGWADA
Tags
Chemistry, Atom, Chemical element

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