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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