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Good quality notes on Physics - Dalton's Atomic Theory. Very helpful for students preparing for Engineering and medical entrance examination and also who are studying in Class XI and XII$2.99
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Good quality notes on Physics - Dalton's Atomic Theory. Very helpful for students preparing for Engineering and medical entrance examination and also who are studying in Class XI and XII
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Course
Physics
Institution
Good quality notes on Physics - Dalton's Atomic Theory. Very helpful for students preparing for Engineering and medical entrance examination and also who are studying in Class XI and XII
Various models for structure of atom
Dalton's Theory
Every material is composed of minute particles known as atom. Atom is indivisible i.e. it cannot be
subdivided. It can neither be created nor be destroyed.
All atoms of same element are identical physically as well as chemically, whereas atoms of different
elements are different in properties.
The atoms of different elements are made up of hydrogen atoms. (The radius of the heaviest atom is
about 10 times that of hydrogen atom and its mass is about 250 times that of hydrogen).
The atom is stable and electrically neutral.
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,TG: @Chalnaayaaar
Thomson's Atomic Model
Part - 02
Thomson's Atom Model
The atom as a whole is electrically neutral because the positive charge present on the atom (sphere)
is equal to the negative charge of electrons present in the sphere. Atom is a positively charged sphere
of radius 10–10 m in which electrons are embedded in between. The positive charge and the whole
mass of the atom is uniformly distributed throughout the sphere.
Electron
Positively
charged
matter
Shortcomings of Thomson's model
(i) The spectrum of atoms cannot be explained with the help of this model
(ii) Scattering of –particles cannot be explained with the help of this model
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, TG: @Chalnaayaaar
Rutherford Atomic Model
Part - 03
Rutherford experiments on scattering of – particles by thin gold foil
Ernst Rutherford, H. Geiger and E. Marsden Performed this experiment.
In this experiment a beam of a-Particles (Helium nucleus) of energy 5.5 MeV, emitted by a 214
83 Bi
radioactive source was directed at a thin metal foil made of gold. They studied the -particles
scattered at various angles.
The experimental arrangement is shown in figure. –particles are emitted by some radioactive material
(polonium), kept inside a thick lead box. A very fine beam of –particles passes through a small hole
in the lead screen. This well collimated beam is then allowed to fall on a thin gold foil. While passing
through the gold foil, –particles are scattered through different angles. A zinc sulphide screen was
placed out the other side of the gold foil. This screen was movable, so as to receive the –particles,
scattered from the gold foil at angles varying from 0 to 180°. When an –particle strikes the screen, it
produces a flash of light and it is observed by the microscope. It was found that :
vacuum ZnS
lead screen
lead box screen gold foil
107m
beam of θ
N() cosec4
-particle
N()
2
most -pass
source of through
-particle
about 1 in 8000 is some are deviated 90 180
repelled back through large angle
• Most of the – particles went straight through the gold foil and produced flashes on the screen as if
there were nothing inside gold foil. Thus the atom is hollow.
• Few particles collided with the atoms of the foil which have scattered or deflected through
considerable large angles. Few particles even turned back towards source itself.
• The entire positive charge and almost whole mass of the atom is concentrated in small centre called
a nucleus.
• The electrons could not deflected the path of a – particles i.e. electrons are very light.
• Electrons revolve round the nucleus in circular orbits. So, Rutherford 1911, proposed a new type of
model of the atom. According to this model, the positive charge of the atom, instead of being
uniformly distributed throughout a sphere of atomic dimension is concentrated in a very small volume
(Less than 10–13m is diameter) at it centre. This central core, now called nucleus, is surrounded by
clouds of electron makes.
Digital [1]
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