ATOMIC STRUCTURE

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ATOMIC STRUCTURE
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Session Objectives

• Daltons theory
• Discovery of fundamental particles
• Thomsons model of an atom
• Rutherfords model
• Concept of atomic number and mass number
• Drawback of Rutherfords model
• Electromagnetic waves
• Plancks quantum theory
• Bohrs model

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Daltons Theory Atom Is Fundamental Particle
Pre 1897

• All matter is composed of atoms.

• All atoms of a given element are identical.

Atom ? Can not be cut.
Indivisible and indestructible
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Cathode rays
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Properties of cathode rays
They are material particles as they produce
mechanical motion in a small paddle wheel
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Properties of Cathode Rays
They are deflected from their path by electric
and magnetic fields
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Anode rays
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Properties of anode Rays
They travel in straight line They are deflected
by electric and magnetic field The nature of
anode rays depends upon the nature of gas
e/m ratio for anode rays is not constant
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Thomsons Model of an Atom
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Rutherford experiment
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Rutherfords Experiment - Results
A beam of ? particles aimed at thin gold foil.
Like firing shells at paper handkerchief with
few of them coming back. - Ernst Rutherford
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Rutherfords Model

• Atom consist of two parts
• NucleusAlmost the whole mass of the atom is
  concentrated in this small region
• (b)Extra nuclear partthis is the space around
  the nucleus in which electrons are revolving at
  high speeds in fixed path

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Concept of atomic mass and atomic number
Atomic number(Z)number of protons
Entire mass of the atom is concentrated at the
centre
Mass number(A)number of protonsnumber of
neutrons
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Concept of atomic number and mass number
For example
Mass numbernumber of protonsnumber of neutrons
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Atomic number number of protons
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Concept of atomic number and mass number
We express weight of an atom in terms of atomic
mass unit (a.m.u).
Mass of a protonMass of neutron 1
a.m.u(approx)
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Drawback of Rutherfords model
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Drawback of Rutherfords model
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Electromagnetic waves
Direction of propagation
Electric and magnetic fields are perpendicular to
each other
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Characteristics of a wave
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Characteristics of a wave
(iii) Velocity The linear distance travelled by
a crest or a trough in one second. Its unit
is cm s-1.
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Electromagnetic spectrum
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Illustrative problem 1
Radio city broadcasts on a frequency of 5,090
KHz.What is the wavelength of electromagnetic
radiation emitted by the transmitter?
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Plancks quantum theory
Radiant energy is emitted or absorbed
discontinuously in the form of quanta.
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Illustrative Problem 2
Solution
Hence, answer is (d).
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Bohrs model
hn
hn
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Bohrs Postulates
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Bohrs model
velocity of electron
radius of various orbits
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Calculation of radius of Bohr orbit
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Calculation of radius of Bohr orbit
centrifugal force
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Calculation of radius of Bohr orbit
For hydrogen Z1
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Calculation of velocity of electron
Dividing (1) by (2),we get
u is in m/s
u is the velocity with which the electron
revolves in an orbit
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Calculation of number of revolutions
Number of revolutions per second
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Calculation of energy of an electron
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We know that
Substituting the value of r we get
P.E. 2K.E. K.E. -Total energy
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Bohrs model
Bohrs postulates
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Illustrative Problem 3
The energy of the electron in the second and
third Bohr orbits of the hydrogen atom is -5.42
X 10-12 and 2.41 X 10-12 respectively.
Calculate the wavelength of the emitted
radiation, when the electron drops from third to
second orbit.
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Solution
According to Plancks quantum theory
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Solution
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Class Exercise - 1
Which of the following fundamentalparticles are
present in the nucleusof an atom?(a) Alpha
particles and protons(b) Protons and
neutrons(c) Protons and electrons(d) Electrons,
protons and neutrons
Solution
The nucleus of an atom is positively charged and
almostthe entire mass of the atom is
concentrated in it. Hence,it contains protons
and neutrons.
Hence, answer is (b).
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Class Exercise - 2
The mass of the proton is(a) 1.672 1024
g(b) 1.672 1025 g(c) 1.672 1025 g(d)
1.672 1026 g
Solution
The mass of the proton is 1.672 1024 g
Hence, answer is (a).
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Class Exercise - 3
Which of the following is not truein case of an
electron?(a) It is a fundamental particle(b)
It has wave nature(c) Its motion is affected by
magnetic field(d) It emits energy while moving
in orbits
Solution
An electron does not emit energy while moving in
orbit.This is so because if it would have done
that it wouldhave eventually fallen into the
nucleus and the atom would have collapsed.
Hence, answer is (d).
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Class Exercise - 4
Positive charge of an atom is(a) concentrated
in the nucleus(b) revolves around the
nucleus(c) scattered all over the atom(d) None
of these
Solution
Positive charge of an atom is present entirely in
thenucleus.
Hence, answer is (a).
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Class Exercise - 5
Calculate and compare the energiesof two
radiations which havewavelengths 6000Å and 4000Å
(h 6.6 x 10-34 J s, c 3 x 108 m s-1)
Solution
3.3 x 10-19 J
4.9 x 10-19 J
0.666 1
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Class Exercise - 6
Why only very few a-particles aredeflected back
on hitting a thingold foil?
Solution
Due to the presence of a very small centre
inwhich the entire mass is concentrated.
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Class Exercise - 7
Explain why cathode rays areproduced only when
the pressurein the discharge tube is very low.
Solution
This is happened because at higher pressureno
electric current flows through the tubeas gases
are poor conductor of electricity.
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Class Exercise - 8
If a neutron is introduced into thenucleus of an
atom, it would resultin the change of(a)
number of electrons(b) atomic number(c) atomic
weight(d) chemical nature of the atom
Solution
Neutrons contribute in a major way to the
weightof the nucleus, thus addition of neutron
wouldresult in increase in the atomic weight.
Hence, answer is (c).
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Class Exercise - 9
The concept of stationary orbits liesin the fact
that(a) Electrons are stationary(b) No change
in energy takes place in stationary
orbit(c) Electrons gain kinetic energy(d)
Energy goes on increasing
Solution
When an electron revolves in a stationary
orbit,no energy change takes place. Energy is
emittedor absorbed only when the electron jumps
fromone stationary orbit to another.
Hence, answer is (c).
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Class Exercise - 10
What is the energy possessed by1 mole of photons
of radiationsof frequency 10 1014 Hz?
Solution
E 6.6 1034 10 1014
E 66 1020 6.6 1019 joules
\ energy of 1 mole of photons 6.6 1019
6.023 1023
39.7518 104
397.518 kJ/mol
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Class test
1.The radius of hydrogen atom in groundstate is
5.3x1011m. It will have a radius of 4.77A after
colliding with an electron. The principal
quantum numberof the atom in the excited state
is (a) 2 (b) 4 (c)3 (d)5
Solution
Hence, answer is (c).
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Electromagnetic waves

• Light is an oscillating
• electro-magnetic field.
• Oscillating electric field
• generates the magnetic
• field and vice-versa.

Electric and magnetic fields are perpendicular to
each other