Atomic Theory

John Dalton, an English schoolteacher, formulated these four postulates of atomic theory during 1803-1807:

1. Each element is composed of extremely small particles called atoms.
2. All atoms of a given element are identical; the atoms of different elements are different and have different properties.
3. Atoms of an element are not changed into different types of atoms by chemical reactions; atoms are neither created nor destroyed in chemical reactions.
4. Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

These postulates then explain some more laws of atomic theory:

1. The law of constant composition: In a given compound, the relative numbers and kinds of atoms are constant.
2. The law of conservation of mass (matter): The total mass of the reactants in a chemical reaction is equal to the total mass of the products.
3. The law of multiple proportions: If 2 elements combine to form different compounds, the ratio of the two elements in the two compounds will be a whole number.

   This theory, however, is slightly flawed. Each atom, in fact, is composed of even smaller particles, protons, neutrons, and electrons. In turn, these are composed of quarks. However, Dalton's theory essentially remains correct and is used in one form by modern Chemists.

   Source: Chemistry: the Central Science, by Brown, LeMay, and Bursten.

An Advance from Sid Meier's Civilization.
Atomic theory, first proposed by Greek philosophers, was revived in the nineteenth century. Advances in physics, especially the development of quantum mechanics, have provided useful (though sometimes bizarre) explanations for the behavior of electrons and other sub-atomic particles.
Prerequisites: Physics and Theory of Gravity. Allows for: Nuclear Fission.

Atomic Theory, a theory as to the existence and properties of atoms; especially, in chemistry, the theory accounting for the fact that in compound bodies the elements combine in certain constant proportions, by assuming that all bodies are composed of ultimate atoms, the weight of which is different in different kinds of matter. It is associated with the name Dalton, who systematized and extended the imperfect results of his predecessors. On its practical side the atomic theory asserts three Laws of Combining Proportions: (1) The Law of Constant or Definite Proportions, teaching that in every chemical compound the nature and proportion of the constituent elements are definite and invariable; (2) The Law of Combination in Multiple Proportions, according to which the several proportions in which one element unites with another, invariably bear towards each other a simple relation; (3) The Law of Combination in Reciprocal Proportions, that the proportions in which two elements combine with a third also represent the proportions in which, or in some simple multiple of which, they will themselves combine. Without expressly adopting the atomic theory, chemists have followed Dalton in the use of the terms atom and atomic weight, yet in using the word atom it should be held in mind that it merely denotes the proportions in which elements unite.

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Entry from Everybody's Cyclopedia, 1912.