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2
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The word ‗atom‘ has been derived from the Greek word ‗a-tomio‘ which means ‗uncutable‘or
‗non-divisible‘
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J
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Thomson, in 1898, proposed that an atom possesses a spherical shape (radius approximately
10–10 m) in which the positive charge is uniformly distributed
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2
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Many different names are given to this model, for example, plum pudding, raisin pudding
or watermelon
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Rutherford’s Nuclear Model of Atom:
a) Most of the space in the atom is empty as most of the a–particles passed through the foil
undeflected
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The deflection must be due to
enormous repulsive force showing that the positive charge of the atom is not spread
throughout the atom as Thomson had presumed
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c) Calculations by Rutherford showed that the volume occupied by the nucleus is negligibly
small as compared to the total volume of the atom
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d) On the basis of above observations and conclusions, Rutherford proposed the nuclear
model of atom (after the discovery of protons)
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This very small portion of the atom was called nucleus by Rutherford
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Thus, Rutherford‘s model of atom resembles the solar
system in which the nucleus plays the role of sun and the electrons that of revolving
planets
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the nucleus is equal
to atomic number (Z )
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e
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Protons and neutrons present in the nucleus are collectively known as nucleons
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mass number (A) = number of
protons (Z) + number of neutrons (n)
6
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On the other hand, atoms with identical atomic number but different atomic mass number

are known as Isotopes
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g
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Drawbacks of Rutherford Model According to the electromagnetic theory of Maxwell, charged
particles when accelerated should emit electromagnetic radiation (This feature does not exist for
planets since they are uncharged)
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The orbit will thus continue to shrink
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But this
does not happen
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5)
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It is
defined as the number of wavelengths per unit length
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e
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9
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Hertz performed a very interesting experiment in which electrons (or electric current) were
ejected when certain metals (for example potassium, rubidium, caesium etc
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The phenomenon is called Photoelectric effect
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Planck’s quantum theory
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(ii) Each packet is called quantum
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The energy of
quantum is directly proportional to the frequency (v) of the radiation
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Its value is 6
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Atoms, molecules or ions that have absorbed radiation are said to be ―excited‖
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A
continuum of radiation is passed through a sample which absorbs radiation of certain
wavelengths
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Line Spectrum of Hydrogen: When an electric discharge is passed through gaseous hydrogen,
the H2 molecules dissociate and the energetically excited hydrogen atoms produced emit
electromagnetic radiation of discrete frequencies
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Series

n1

n2

Spectral region

Lyman

1

2,3……

Ultraviolet

Balmer

2

3,4………

Visible

Paschen

3

4,5……
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Infrared

Pfund

5

6,7……
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Bohr’s Model For Hydrogen Atom
a) The electron in the hydrogen atom can move around the nucleus in a circular path of fixed
radius and energy
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These orbits are arranged concentrically around the nucleus
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The angular
momentum of an electron in a given stationary state can be expressed as in equation

c) The energy of an electron in the orbit does not change with time
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The energy change does not take place
in a continuous manner
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Bohr‘s theory can also be applied to the ions containing only one electron, similar to that present
in hydrogen atom
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The energies of the stationary states
associated with these kinds of ions (also known as hydrogen like species) are given by the

expression
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This model is also unable to explain the spectrum of atoms other than hydrogen, for
example, helium atom which possesses only two electrons
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e
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18
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:de Broglie relation state that the wavelength associated with a moving
object or an electron is inversely proportional to the momentum of the particle
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mv
p

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It is not possible to determine the position and velocity
simultaneously for a sub-atomic particle like electron at any given instant to an arbitary degree of
precision
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If
‗  x‘ is uncertainty in position and ‗  P‘ is uncertainty in momentum then
x
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Orbital
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They are used to get complete information about electron, i
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, location,
energy, spin, etc
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22
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It specifies the location and energy of an electron
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It is denoted by ‗n‘
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It is also called ‗azimuthal quantum number‘
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It is denoted by ‗l‘
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, upto n–1
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e
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, if value of n is 4, l can have values
0, 1, 2, 3
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mvr  l (l  1)

h
2

It is denoted by ‗m‘ and its value depends on value of ‗l‘ since

magnetism is due to angular momentum
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e
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Its permitted values
are - l to + l including zero, e
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, when l = 1, then m = -1, 0, +1
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It indicates, the direction in which electron revolves
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It has two permitted values + ½ or – ½
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In case two sub-shell has equal value of (n+l),
the sub-shell with lower value of n has lower energy
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No two electrons in an atom can have all the four quantum
numbers same
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Electrons are filled in the various orbitals in the increasing order of their
energies, i
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, orbital having lowest energy will be filled first and the orbital having highest
energy will be filled last
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Hund’s rule of maximum multiplicity
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g
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It has been found that there is extra stability(Stability of
Completely Filled and Half Filled Subshells) associated with these electronic configurations
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Three orbitals of 2p subshell (2px, 2py, and 2pz orbitals)
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Five orbitals of 3d subshell (3dxy, 3dyz, 3dzx 3dx2-y2and 3dx2 orbitals)

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