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 atoms, derived from the Greek word ‘atomos’ that 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’.

“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.

“The modern researches have clearly shown that an atom is further composed of smaller particles called subatomic particles.”

Examples: Electron, proton, neutron, hypron, neutrino, anti-neutrino, etc.

More than 100 such particles are thought to exist in an atom.

Electron, proton and neutron are regarded as the fundamental particles of atom. This is because they present in every atom. However, there is one exception, Protium, the isotope of hydrogen, doesn’t have a neutron in the nucleus.

J. Berzelius, the Swedish chemist:

(1) Determined the atomic masses of elements. A number of his values are close to the modern values of atomic masses.

(2) Developed the system of giving element a symbol.

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