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The Atom: From Philosophical idea to
Scientific Theory
The particle theory of matter was supported as early as 400 B
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by certain Greek thinkers, such as
Democritus
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”
Aristotle was part of the generation that succeeded Democritus
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Neither the view of Aristotle nor that of
Democritus was supported by experimental evidence, so each remained speculation until the eighteenth
century
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Foundations of Atomic Theory
Virtually all chemists in the late 1700s accepted the modern definition of an element as a substance that
cannot be further broken down by ordinary chemical means
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There was great controversy, however, as to whether elements always combine in the same
ratio when forming a particular compound
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In the 1790s, the study of matter was revolutionized by a new emphasis on the
quantitative analysis of chemical reactions
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This led to the
discovery of several basic laws
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This
discovery was soon fooled by the assertion that, regardless of where or how a pure chemical compound
is prepared, it is composed of a fixed proportion of elements
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34% by mass of the element sodium, Na, and 60
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The fact that a chemical compound contains the same elements in exactly
the same proportions by mass regardless of the size of the sample or source of the compound is known
as the law of definite proportions
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For
example, the elements carbon and oxygen form two compounds, carbon dioxide and carbon monoxide
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00 g of carbon
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66
g of oxygen combine with 1
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In carbon monoxide, 1
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00 g
of carbon
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66 to 1
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This
illustrates the law of multiple proportions: If two or more different compounds are composed of th
same two elements, then the ratio of the masses of the second element combined with a certain mass of
the first element is always a ratio of small whole numbers

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