Monday, 25 June 2012

History of an ATOM with various theories and models on it...!!

History of Atoms

For Hundreds of Years, Scientists were curious to know about the smallest part of "Matter". In the very early years of B.C. RISHI KANAD said that every object is made up of indivisible tiny particles he called them as "PARMANU".

In 430 B.C. a Greek Philosopher Democritus postulated that matter is made up of very small particle called " Atomos " which means " uncuttable " or
" the smallest indivisible particle of matter ". According to Democritus if we break a piece of matter into two half, and then again break it into two half, then how many breaks we have to make so that we cannot break it further? Democritus thought that it ended at some point, and that smallest possible part of matter which cannot be further divided is called as Atom. This concept of Democritus was declared worthless by Aristotle (a Greek Philosopher and teacher of Alexander the Great) whose opinion was considered very important by the people at that time.

After this for more than 2000 years nobody did anything to continue the research started by the Greeks.

Then in 1803 A.D. John Dalton an English Chemist (wellknown for his work in the development of Modern Atomic Theory and his research in colour blindness) put forward his theory.

Dalton's Atomic Theory :-
1) Matter is made of atoms. Atoms are indivisible and indestructible.
2) All atoms of a given element are identical in mass and properties.
3) Compounds are formed by a combination of two or more different kinds of atoms.
4) A chemical reaction is a rearrangement of atoms. However it had some Drawbacks and was rejected.

Sir Joseph John Thomson Theory :-
After John Dalton,In 1867 Sir Joseph John Thomson a British physicist( who was awarded Nobel Prize in Physics in 1906 for the discovery of the electron and for his work on the conduction of electricity in gases) developed another theory.
The theory said that :-
1) An atom is a sphere of negatively charged electrons and positively charged protons which are uniformly mixed throughout the atom.
2) Atoms as a whole were neutrally charged as the amount of positive charge in it cancels the amount of negative charge on electrons.
3)The electrons are randomly scattered throughout the atom.
4)The electrons can be moved around or taken out of the atom, but the positive charge cannot.
However the so called " plum pudding model " of J. J. Thomson of the atom was incorrect and needed some corrections.
Till now the components of the atom i.e protons and electrons had been discovered but it was not clear how these protons and electrons were arranged within the atom. 

Rutherford's Theory :-
After J.J. Thomson, Rutherford in 1911 made his Atomic Model which had
certain limitations. According to his so called "the Planetary Model of the atom" :-
1) An atom has a minute and positively charged body at its centre known as nucleus.
2) All the protons and neutrons are in the nucleus and the electrons orbited around the nucleus like planets around the sun.

He further explained the reason for electrons not falling in to the nucleus in spite of the mutual electrostatic forces of attraction between the negatively charged electrons outside and the positively charged protons inside the nucleus. According to him the electrons are revolving at such a high velocity that the centrifugal force acting outwards balances the electrostatic forces of attraction acting inwards and due to this, the electrons do not fall in to the nucleus.
However the Rutherford's atomic model had some limitations and needed
some modifications. 

Niel Bohr Theory :-
In 1922 a Danish physicist Niel Bohr was the one who pointed out a major mistake in Rutherford's atomic model on the basis of J.C. Maxwell theory.He revised Rutherford's atomic model based on quantum mechanics.

Main Points of the Bohr Model are :-
1)The electrons of an atom revolve around the nucleus in orbits.
2)The electrons can only travel in certain orbits called by Bohr as the "stationary orbits" which are at a certain discrete set of distances from the nucleus with specific energies.
3)The electrons do not continuously lose energy as they travel in a particular orbit.
4)The electrons lose energy only when they jump from one orbit to another, absorbing or emitting electromagnetic radiation.
The Bohr's Model too had problems but it is important because it describes most of the accepted features of atomic theory.

This is all about the various theories and models related to atom from the beginning.This is how as the years passed our vision of atom became clearer.

Thank you for visiting the blog...!!

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