11th Class Chemistry Book in pdf download

Atom is the smallest particle of an element, which can take part in a chemical reaction. For

example, He and Ne, etc. have atoms, which have independent existence while atoms of hydrogen,

nitrogen and oxygen do not exist independently.

Long time ago, it was thought that matter is made up of simple, indivisible particles. Greek

philosophers thought that, matter could be divided into smaller and smaller particles to reach

a basic unit, which could not be further sub-divided. Democritus (460-370 B.C.) called these

particles Atomos, derived from the word “Atomos” means indivisible. However, the ideas of Greek

philosophers were not based on experimental evidences.

In the late 17th century, the quantitative study of the composition of pure substances disclosed

that a few elements were the components of many different substances. It was also investigated

that how, elements combined to form compounds and how compounds could be broken down

into their constituent elements.

In 1808, an English school teacher, John Dalton, recognized that the law of conservation of matter

and the law of definite proportions could be explained by the existence of atoms. He developed

an atomic theory; the main postulate of which is that all matter is composed of atoms of different

elements, which differ in their properties.

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