STRUCTURE OF ATOM
Subatomic Particles
(i) Symbol
(ii) Approximate relative mass
(iii) Mass in kg
(iv) Mass in amu
(v) Charge (coulomb)
(vi) Actual Charge (e.s.u)
ATOMIC MODELS
Thomson’s Atomic Model (Plum – pudding model):
Postulate: Atom is a sphere of positive charge in which number of electrons are embedded.
Limitations:  Could not satisfactorily explain the results of scattering experiment carried out by Rutherford.
RUTHERFORD’S MODEL
Postulates:
Limitations:  Could not explain stability and electronic structure of atom.
ATOMIC TERMS
(i) Atomic Number (Z)
(ii) Mass Number (A)
(iii) Nucleons
(iv) Isotopes
(v) Isobars
(vi) Isotones
(vii) Isoelectronic
(viii) Nuclear isomers
(ix) Isosters
WAVE
(i) Wave length (λ)
(ii) Frequency (ν)
(iii) Velocity (c)
(iv) Wave number
(v) Amplitude (a)
ELECTROMAGNETIC WAVES
Radiations  Wave length (Å) 
Radio waves  3×10^{14} to 3 ×10^{7} 
Micro waves  3×10^{9} to 3 ×10^{6} 
Infrared (IR)  6×10^{6} to 7600 
Visible  7600 to 3800 
Ultra violet (UV)  3800 to 150 
X–rays  150 to 0.1 
Gamma rays  0.1 to 0.01 
ATOMIC SPECTRUM OF HYDROGEN ATOM
Where, RH = Rydberg constant (108978 cm^{1})
n_{1} and n_{2} have integral values as follows
n_{1}  n_{2}  Spectral Series  Spectral region 
1  2,3,4…  Lyman  UV 
2  3,4,5…  Balmer  Visible 
3  4,5,6…  Pascher  IR 
4  5,6,7…  Brackett  IR 
5  6,7,8…  Pfund  IR 
Photoelectric effect:
Planck’s quantum theory:
Substances radiate or absorb energy discontinuously in the form of energy packets
The smallest packet of energy is called quantum. In case of light the quantum is known as photon.
The energy of a quantum is directly proportional to the frequency of the radiation.
E = hv were v is the frequency of radiation and h is Planck’s constant having the value
6.626 × 10^{–27} erg sec or 6.626 × 10^{–34 }J sec.
A body can radiate or absorb energy in whole number multiples of quantum hn, 2hν,3hν………..nhν, where n is the positive integer.
Bohr’s atomic model:
Limitations of Bohr’s theory:
de Broglie equation:
λ = h/mv = h/p
Heisenberg’s uncertainty principle:
It is impossible to determine simultaneously, the exact position and exact momentum of an electron.
QUANTUM NUMBERS
Principal quantum number (n):
Azimuthal or angular momentum quantum number (l):
Value of l  0  1  2  3  4 
Notation of sub shell  s  p  d  f  g 
The magnetic quantum number (m):
Value of l  0  1  2  3 
Notation of sub shell  s  p  d  f 
Values of m  0  1,0,1  2,2,0,1,2  3,2,1,0,1,2,3 
The spin quantum number (s)
RULES FOR FILLING OF ELECTRONS IN VARIOUS ORBITALS
Aufbau Principle:
Pauli’s Exclusion principle:
An orbital can contain a maximum number of two electrons and these two electrons must be of opposite spin.
Hund’s rule of maximum multiplicity:
Electron pairing in p, d and f orbital cannot occur until each orbital of a given subshell contains one electron each or is singly occupied”.
EXCEPTIONAL CONFIGURATIONS
Stability of half filled and completely filled orbitals
Cu has 29 electrons. Its expected electronic configuration is
1s^{2}, 2s^{2}, 2p^{6}, 3s^{2}, 3p^{6}, 4s^{2}, 3d^{9}
But a shift of one electron from lower energy 4s orbital to higher energy 3d orbital will make the distribution of electron symmetrical and hence will impart more stability.
Thus the electronic configuration of Cu is
1s^{2}, 2s^{2}, 2p^{6}, 3s^{2}, 3p^{6}, 4s^{1}, 3d^{10}
Fully filled and half filled orbitals are more stable
1. What is the structure of an atom? 
2. How are protons and neutrons arranged in the nucleus of an atom? 
3. What are energy levels or shells in an atom? 
4. How are electrons distributed in the energy levels of an atom? 
5. What is the significance of the structure of an atom in understanding its chemical properties? 

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