Atoms and Molecules

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Existence of atoms

The strongest evidence for the reality of atoms comes from scanning tunneling microscopy, studies of Brownian motion, and by the remarkably good agreement for experimental estimates of Avogadro's Number.

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History of atoms

The idea of divisibility of atoms goes back to around 500 B.C. An Indian philosopher 'Maharishi Kanad' was the first one who postulated that the atom is indivisible. He named these particles 'Parmanu'. Another greek philosophers "Democritus and Leucippus" suggested that if we go on dividing matter, a time will come that we cannot further divide. He named those particles as atoms. Antonie Lavoisier laid the foundation of chemical sciences by giving the two important laws of chemical combination.
1. Laws of conservation of mass
2. Law of definite proportion

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Molecules of Elements

A molecule is the fundamental building block of a substance which can normally exist on its own. When two or more atoms of same type combine, the molecule of an element is obtained.
Examples of molecules of element are Hydrogen, Oxygen etc. 

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Molecules of Element

A molecule is the smallest part of a substance which can normally exist on its own. Atoms of the same element combine to form a molecule of an  element. A molecule is known for retaining the physical and chemical properties of the substance.

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Structure of atom and nucleus

Structure of atom: An atom consists of a nucleus at its centre, surrounded by electrons which are revolving in some specific stationary shells.
Structure of nucleus: The nucleus is at the centre whose size is of the order of m  to  m. It consists of protons and neutrons.