chap2 - Chapter 2 Atoms, Molecules, and Ions 2.1 The Atomic...

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Chapter 2 – Atoms, Molecules, and Ions 2.1 The Atomic Theory of Matter Throughout most of history, matter was believed to be infinitely divisible. That is, there was no smallest particle of anything. While Democritus proposed the concept of the atom centuries earlier, it was not until 1803 that John Dalton proposed the modern concept of the atom, which consisted of 4 empirically based postulates: 1) All elements are composed of extremely small particles called atoms. 2) All atoms of a particular element are identical. Atoms of different elements are different. Atoms of the same element have identical properties, while atoms of different elements have different properties. 3) Atoms of different elements cannot be interconverted [by chemical reactions]. Neither can they be created or destroyed. 4) Compounds are formed from atoms of different elements in fixed ratios. With one minor caveat that we will get to later, these rules are as valid today as they were over 200 years ago. Thus, the atom is the smallest unit of an element that retains the chemical identity of that element. In a sense, it also represents the dividing line between chemistry and physics. Generally, chemists study atoms and larger species, while physics study particles smaller than the atom. The atomic theory was important for a number of reasons. First, it explained a number of well-established principles. For example, the law of conservation of matter states that in a chemical reaction the sum of the masses of the reactants must equal the sum of the masses of the products. In other words, the weight of what comes out of a chemical reaction must equal the weight of what went in. Postulate 3 explains this observation. The Atomic Theory also represents the dividing line between chemistry and alchemy. You
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2 know of alchemy as the attempt to turn base (inexpensive) metals into valuable metals (e.g. gold). The Atomic Theory recognized this as impossible (Postulate 3). Shortly after publication of the Atomic Theory, alchemy, which was already in decline, disappeared from western science. 2.2 The Discovery of Atomic Structure While chemists embraced the concept of the atom relatively quickly, physicists did so more slowly, and some did not accept them until after the turn of the 20th century. Why? Because no direct evidence existed for them. Chemists thought of atoms as much like billiard balls: hard and indivisible. If so, no direct evidence for them could exist with the technology of the time. Physicists initially thought of atoms as a convenient hypothetical construct, but not real entities. By the late 1800s, electricity had been known for quite sometime (e.g. Benjamin Franklin’s famous 1752 experiment). The existence of charge (positive and negative) was accepted, although its origin was not understood. This represented a major question for physicists. Physicists knew that passing an electrical current between two metal plates in an evacuated
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This note was uploaded on 02/12/2012 for the course CHM 211 taught by Professor Staff during the Fall '08 term at Marshall.

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chap2 - Chapter 2 Atoms, Molecules, and Ions 2.1 The Atomic...

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