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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