Structure of Atom

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A project to elaborate the discovery and
advancement in the field of existence of
atom and its various fundamental particles
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After this animation you will be
able to
 Know more about the discovery of electron , proton
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and neutron.
Thomson, Rutherford and Bohr atomic models.
Rutherford’s Alpha scattering experiment.
Quantum mechanical model of atom.
Photoelectric effect.
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Discovery of atom
 The existence of atom has been proposed since the
time of early Indian and Greek philosophers(400 B.C.)
who were of the view that atoms are the fundamental
building blocks of matter.
 The word atom has been derived from the Greek word
“a-tomio” which means uncut able or non-divisible.
These ideas remain dormant for a very long time and
were revived again by scientists in nineteenth century.
 Scientists work hard to discover the sub-atomic
particles and those experiments are as….
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Discharge Tube Experiment
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 William Crooks performed this exp. In cylindrical glass tube as
shown in figure.
 Observations :
1. No current flows at 1 atm pressure even at high voltage (about
104 V )
2. When pressure is reduced to 10-2 atm, gas is found to emit light
which depends upon the nature of gas.
3. Further decrease in pressure stops emission of light but walls
opposite to cathode starts glowing and this is called
fluorescence.
 Result : Fluorescence is due to rays emitted from cathode and
hence these rays are called cathode rays.
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Properties of cathode ray
J.J.Thomson studied the important properties of cathode
rays as follows:
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 Cathode rays travel in a straight line .
 Cathode rays are made up of material particles.
 Cathode rays consists of negatively charged particles.
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Discovery of electrons
 As cathode rays are made up of material particles which
are attracted towards the positively charged plate. So ,
cathode rays consists of negatively charged particles and
they were named as Electrons.
 As J.J.Thomson studied the properties of cathode rays
which led to the discovery of electrons so, its
J.J. Thomson who discovered electrons
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Discovery of Protons( Anode Rays)
 In 1886, Goldstein performed discharge tube experiment with certain
modifications as shown in figure.
 He observed new type of luminous rays passing through perforations of
cathode and moving in directions opposite to cathode rays. So, these
rays consists of positively charged particles called Protons and these
rays are called Anode rays.
 Anode rays are not emitted from anode but from a space between
anode and cathode.
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J.J. Thomson model of Atom
 In 1904, Thomson proposed that atom is a sphere of positively
charged particles in which negatively charged electrons are
embedded. Stability of atom was explained on the basis of
attraction between positively and negatively charged protons
and electrons respectively.
 Drawbacks: One major drawback is that it could not explained
the Rutherford’s Alpha Scattering Experiment , hence rejected.
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Rutherford’s α-Scattering Experiment
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 Observation:
Most of the α-particles passed through the gold foil.
2. A small fraction of α-particles was deflected by small
angles.
3. A very few α-particles (~1 in 20,000)bounced back,
that is, were deflected by nearly 1800.
1.
For Conclusions Click
Here
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Drawbacks of Rutherford’s Model
Of An Atom
 His model cannot explain the hydrogen atomic spectra
and of other elements.
 When an electron revolves in a orbit the it undergoes
acceleration and due to which it looses energy and
hence ultimately fall into the nucleus. But actually this
does not happened so his model is not appropriate.
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Planck Quantum Theory
After the failure of Electromagnetic wave theory, Max.Planck
came into picture and gives few postulates as:
1. Radiant energy is emitted or absorbed not continuously but discontinuously in
the form of packets called Quantum. & these quantum a re called Photons in case
of light.
2. Energy of each quantum is directly proportional to the frequency of radiation. i.e.
E ∞ V or E = hv
where h is planks constant such that
h = 6.626 X 10-34 J/s
3. Total amount of energy emitted is whole number multiple of quantum , i.e.
E = nhv where n in any natural number
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Photoelectric effect
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 DEFINITION : The ejection of electrons from the surface of a metal under the
influence of striking photons .
 EXPLANATION : Actually electrons are held by the nuclei by some force called
Binding Energy. But photons which fall on the carries energy and hence excite
them to the surface from where electrons start moving in a definite direction with
definite amount of kinetic energy.
 CONDITIONS :
Energy of incident photons = Work Function + Kinetic
Energy of electron
hυ = hυ0 + ½ mv2
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Bohr’s Model Of An Atom
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 Whole mass of atom is present in the central core
called nucleus.
 Electrons revolve around the nucleus in stationary
orbits also called energy levels.
 Angular momentum of an electron in a given
stationary orbit can be given by
mvr = nh/2π
 No. of electrons in a given energy level is given by
No of electrons = 2n2
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Failures Of Bohr’s Model
 The theory could not explain the atomic spectra of atoms
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containing more than one electrons.
Theory failed to explain the fine structure of spectral lines.
Splitting of lines in the magnetic field is known as Zeeman
Effect and splitting of lines in the electric field is known as
Stark Effect. Bohr’s theory could not offer any satisfactory
explanation of these effects.
Theory failed to explain the shapes of the molecules
formed by the combination of atoms.
Bohr’s Theory could not explain the de Broglie’s
Relationship and Heisenberg’s Uncertainty Principle.
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De Broglie Concept
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 He stated that moving particles are associated with
dual nature i.e. wave and particle nature.
 He co-related the two characters in the form of an
equation known as de-Broglie Equation.
λ = h/mv
Heisenberg’s Uncertainty Principle
 He stated that, It is not possible to measure
simultaneously the position and the momentum of a
microscopic particle with absolute accuracy or
certainty.
 Mathematically,
∆x.∆p = h/4π
 This principle is a prove that electron can never exists
inside the nucleus.
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Quantum Mechanical Model Of An
Atom
 It is found that instead of revolving in an orbit
electrons actually revolve in orbital and these orbital
aggregate to form orbits.
 To know more Click Here
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