Transcript Atomic Model - Kendriya Vidyalaya Churu

THOMSON MODEL OF ATOM
* J.J THOMSON in 1898,he
proposed that an atom
possesses a spherical shape
(radius approximately 10
raise -10m) in which the
positive charge is uniformly
distributed.
*The electrons are embedded into
Sir J.J Thomson
it in such a manner as to give the
most stable electrostatic
arrangement . Many different
names are given to this model ,for
example , plum pudding, raisin
pudding or watermelon .This model
can be visualized as a pudding or
watermelon of positive charge with
plums or seeds (electrons)
embedded into it . An important
feature of this model is that the
mass of the atom is assumed to be
uniformly distributed over the
atom. Although this model was
able to explain the overall
neutrality of the atom ,but was not
consistent with the results of later
experiment.
RUTHERFORD’S NUCLEAR MODEL OF
ATOM
According to this model:1) The positive charge and most of the mass of
the atom was densely concentrated in
extremely small region . This very small portion
of the atom was called nucleus .
2) The nucleus is surrounded by electrons that
move around the nucleus with a very high
speed in circular paths called orbits.
3) Electrons and the nucleus are held together by
electrostatic forces of attraction.
RUTHERFORD’S SCATTERING
EXPERIMENT
SCHEMATIC MOLECULAR VIEW OF
THE GOLD FOIL
OBSERVATIONS:1)
Most of the alpha particles passed through the gold foil
undeflected .
2) A small fraction of the alpha particles was deflected by small
angles .
3) A very few alpha particles (approx. 1 in 20,000) bounded back
,that is, were deflected nearly 180 degree.
1) Most of the space in the atom is empty as most of the alpha particles
passed through the foil undeflected .
2) A few positively charged alpha particles were deflected . The deflection
must be due to enormous repulsive force showing that the positive of the
atom is not spread throughout the atom as Thomson had presumed the
positive charge has to be concentrated in a very small volume that
repelled and deflected the positively charged alpha particle .
3) Calculation by Rutherford showed the volume occupied by the nucleus
is negligibly small as compared to the total volume of the atom. The
radius of the atom is about 10 raise -10m , while that of nucleus is 10 raise 15m .one can appreciate this difference in size by realizing that if a cricket
ball represents in a nucleus than the radius of the atom would be about
5km.
OBSERVATION AND CONCLUSION, RUTHERFORD PROPOSED
THE NUCLEAR MODEL OF ATOM (AFTER THE DISCOVERY OF
PROTON). ACCORDING TO THIS MODEL :The positive charge and most of the mass of the atom was
densely concentrated in extremely small region . This very
small portion of the atom was called nucleus by Rutherford.
2) The nucleus is surrounded by electrons that move around the
nucleus with a very high speed in circular paths called orbits. Thus
Rutherford’s model of atom resembles the solar system in which
the nucleus plays a role of sun and the electrons that of revolving
planets .
3) Electrons and the nucleus are held together by electrostatic
forces of attraction.
1)
1)
2)
3)
It could not explain stability of an atom .
According to eletromagnetic theory it charge
body is present in a motion in it should be emits
some radiation finally it should fall in the
nucleus .
He could not explain about electronic structure
of an atom.
1)
Dual character of the electromagnetic
radiation which means that radiation possess
both wave like and particle like properties .
2) Experimental results regarding atomic spectra
which can be explained only by assuming
quantized eletronics levels in atoms .
* DEVELOPMENT LEADING
TO THE BOHR’S MODEL
OF ATOM
Electrons are revolve around the nucleus in fixed
energy circular path known as orbits.
2) The energy of electrons does not changed until it
remains same orbit if it absorb energy a jump of higher
orbit and it returned to original state then it emit
energy .
3) A frequency of radiation absorbed or emitted is
equal to , V = ∆E / h .
4) The angular momentmum of an electrons is an
integral multiple of :- mvr = nh/ 2∏.
1)
LIMITATIONS OF BOHR’S
MODEL
It could be explained only he could
explained single electron system spectrum .
2) He could not explained splitting of lines in the
presence of magnetic field known as zeeman,
and electric field known as stark .
3) He could not explain fines spectra of
hydrogen .
if an electron completely remove then
the energy known as ionization.
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MADE BY :-
SAPNA – 22
MINAKSHI -14
GULSHAD-8
GITANJALI-7
CLASS- 11 SCIENCE