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Dalton's Atomic Theory

In 1803, John Dalton proposed the atomic theory of matter. The main postulates of his atomic theory can be summarized as follows:
  1. Matter is composed of indivisible particles - atoms.
  2. An element is composed of only one kind of atom. These atoms in a particular element have the same properties such as mass, size, or even shape.
  3. A compound is composed of two or more elements combined in fixed ratios or proportions.
  4. In a chemical reaction, the atoms in the reactants recombine, resulting in products which represent the combination of atoms present in the reactants. In the process, atoms are neither created, nor destroyed. So a chemical reaction is essentially a rearrangement of atoms.

Ramifications of Dalton's Theory

The atomic theory put forward by Dalton is consistent with the law of conservation of mass. As the fourth postulate says, chemical reaction is just a rearrangement of atoms, and thus the total mass remains constant during a chemical reaction.
The postulates also account for the law of definite proportions. Compounds are made of elements in fixed or definite proportions. Since the atoms have fixed mass, compounds should have elements in a fixed ratio with respect to mass. Finally, these postulates predict what is known as the law of multiple proportions. According to this law, if two elements form two or more different compounds, the ratio of the mass of one element of these compounds to a fixed mass of the other element is a simple whole number.

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