In the fifth century (B.C.) the Greek Philosopher Democritus expressed the belief that all matter consists of very small indivisible particles, which he named ATOMOS (Greek =Atoms = not cut able = indivisible) nowadays called atoms.

3.1.1 Daltons Atomic Theory:

In (1808) John Dalton, an English school teacher and Chemist suggested the fundamental ideas of atomic theory, which explains the chemical nature of matter and the existence of atoms. It is known as Dalton 's atomic theory. The important postulates are as under:

Indestructible particles called atoms.

•  All, elements are made up of small indivisible, indestructible particles called atoms.

•  All atoms of a given element are identical in all respects, having same size, mass and chemical properties. But the atoms of one element differ from the atoms of other element.

•  Compounds are formed when atoms of more than one element combine in a simple whole number ratio.

•  A chemical reaction is a rearrangement of atoms, but atoms themselves are not changed, this means that atoms are neither created nor destroyed in chemical reactions.

3.1.2 Modern Atomic Theory:

Daltons atomic theory assumed that atoms of elements are indivisible ' and that no particles smaller than atoms existed. But as the time passed new experimental facts led to the modification and extension of Dalton 's atomic theory. Atom is a complex organization, composed of even smaller particles called sub-atomic particles (fundamental particles). These are electrons, protons and neutrons. Dalton 's view that all atoms of an element have the same mass is modified in the light of discovery of isotopes. Even then, we can say that the Dalton 's atomic theory was largely successful in explaining the laws of chemical combinations.

3.1.3 Fundamental Particles of an Atom:

The atom was generally identified as the smallest particle of an element, consisting of sub atomic particles, the electrons, protons and neutrons. The first hint about the sub-atomic particles came with the discovery of electron by M. Faraday (1832), William Crooks (1879) and J-J. I Thomson (1897). Later, the second sub-atomic particle, the proton was identified and isolated by Goldstein, German scientist (1886) and Ernest Rutherford (1919). Finally an English scientist James Chadwick revealed the third particle the neutron in 1932. The structure of atom as we know it today, is because of these findings.


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