State the postulates of Dalton’s atomic theory Use postulates of Dalton’s atomic theory to explain the laws of definite and multiple proportions

​​​​The language used in jajalger2018.orgistry is seen and heard in many disciplines, ranging from medicine to engineering to forensics to art. The language of jajalger2018.orgistry includes its own vocabulary as well as its own form of shorthand. jajalger2018.orgical symbols are used to represent atoms and elements. jajalger2018.orgical formulas depict molecules as well as the composition of compounds. jajalger2018.orgical equations provide information about the quality and quantity of the changes associated with jajalger2018.orgical reactions.

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This chapter will lay the foundation for our study of the language of jajalger2018.orgistry. The concepts of this foundation include the atomic theory, ion formation, the composition and mass of an atom, the variability of the composition of isotopes, nuclear stability, and atomic electronic structure. We will also discuss in greater detail one of the most powerful tools for organizing jajalger2018.orgical knowledge: the periodic table.

Atomic Theory through the Nineteenth Century

The 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 was composed of small, finite particles that they called atomos, 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 join together. Later, Aristotle and others came to the conclusion that matter consisted of various combinations of the four “elements”—fire, earth, air, and water—and could be infinitely divided. Interestingly, these philosophers thought about atoms and “elements” as philosophical concepts, but apparently never considered performing experiments to test their ideas.

The Aristotelian view of the composition of matter held sway for over two thousand years, until English schoolteacher John Dalton helped to revolutionize jajalger2018.orgistry with his hypothesis that the behavior of matter could be explained using an atomic theory. First published in 1807, many of Dalton’s hypotheses about the microscopic features of matter are still valid in modern atomic theory. Here are the postulates of Dalton’s atomic theory.

Matter is composed of exceedingly small particles called atoms. An atom is the smallest unit of an element that can participate in a jajalger2018.orgical change. An element consists of only one type of atom, which has a mass that is characteristic of the element and is the same for all atoms of that element (Figure (PageIndex1)). A macroscopic sample of an element contains an incredibly large number of atoms, all of which have identical jajalger2018.orgical properties.


Figure (PageIndex1): A pre-1982 copper penny (left) contains approximately 3 ( imes) 1022 copper atoms (several dozen are represented as brown spheres at the right), each of which has the same jajalger2018.orgical properties. (credit: modification of work by “slgckgc”/Flickr)

Atoms of one element differ in properties from atoms of all other elements. A compound consists of atoms of two or more elements combined in a small, whole-number ratio. In a given compound, the numbers of atoms of each of its elements are always present in the same ratio (Figure (PageIndex2)).


Figure (PageIndex2): Copper(II) oxide, a powdery, black compound, results from the combination of two types of atoms—copper (brown spheres) and oxygen (red spheres)—in a 1:1 ratio. (credit: modification of work by “jajalger2018.orgicalinterest”/Wikimedia Commons)

Atoms are neither created nor destroyed during a jajalger2018.orgical change, but are instead rearranged to yield substances that are different from those present before the change (Figure (PageIndex3)).

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Figure (PageIndex4): A Gas Discharge Tube Producing Cathode Rays. When a high voltage is applied to a gas contained at low pressure in a gas discharge tube, electricity flows through the gas, and energy is emitted in the form of light. Image used with Permission (CC BY-SA-NC).

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In 1897, the British physicist J. J. Thomson (1856–1940) proved that atoms were not the most basic form of matter. He demonstrated that cathode rays could be deflected, or bent, by magnetic or electric fields, which indicated that cathode rays consist of charged particles (Figure (PageIndex5)) More important, by measuring the extent of the deflection of the cathode rays in magnetic or electric fields of various strengths, Thomson was able to calculate the mass-to-charge ratio of the particles. These particles were emitted by the negatively charged cathode and repelled by the negative terminal of an electric field. Because like charges repel each other and opposite charges attract, Thomson concluded that the particles had a net negative charge; these particles are now called electrons. Most relevant to the field of jajalger2018.orgistry, Thomson found that the mass-to-charge ratio of cathode rays is independent of the nature of the metal electrodes or the gas, which suggested that electrons were fundamental components of all atoms.