History of Atomic Theory
Ancient Greek Origins
The idea of the atom was first proposed by the Greek philosophers Democritus and Leucippus around 400 B.C. At that time, there was absolutely no real evidence that supported this proposal. Even after 20 centuries later, no experiment was strong enough to verify the existence of the atom.
18th Century Developments
In the 18th Century, the first scientific data on the atom were gathered by A. L. Lavoisier and others from quantitative measurements of chemical reactions. From the experiment, he suggested that there exist some elements which could not be disintegrated into any smaller composition by usual chemical method. He defined this as chemical element.
Dalton's Atomic Theory
From the results of Lavoisier experiments, John Dalton proposed the first systematic atomic theory. This theory of the atom is composed of two basic chemical laws: the law of constant proportions and the law of multiple proportions.
Law of Constant Proportions
"The composition of a pure chemical compound is independent of its method of preparation"
Example: Water is a compound of hydrogen and oxygen. The ratio of the weight of hydrogen to oxygen in water is fixed at the value 1:8, independent of how it is formed.
Law of Multiple Proportions
"When two elements A and B combine to form more than one compound, the weights of B which combine with a fixed weight of A are in the proportion of small whole numbers (integers)"
Example: Carbon and oxygen react to form CO or CO₂ but not CO₁.₁ or CO₁.₂
