atom model

1. Bohr atom model
‘’An electron cannot revolve around the
nucleus in all possible orbits”
-Classical theory
Bohr’s postulates
1.The electron cannot revolve around the
nucleus only in those allowed or permissible
orbits for which the angular momentum the
integral multiple of h.
2.An atom radiates energy when an
electron jumps from a stationary orbit of
higher energy to one of the orbit of energy
and the photon is emmitted

2. By using Bohr formulae we can calculate
a)radii of the stationary orbits
b)total energy of the electron in the orbit
SPECTRAL SERIES OF THE HYDROGEN ATOM:
a) Lyman series
b) Balmer series
c) paschen series
d) Brackett series
e) Pfund series

3. CRITICAL POTENTIALS:
The least energy expressed in electron volts
required to excite a free neutral atom from its
ground state to a higher state is called critical
potential
EXCITATION POTENTIALS:
The energy in electron volts is required to
raise an atom from its normal state into an excited
state is called potential of the state
IONISATION POTENTIALS:
The energy required to remove an electron
from a given orbit to an infinite distance from the
nucleus

4. Experimental determination of critical
potentials
Frank and Hertz’s method
This experiment shows that the energy lost by the
electron in its collision with the mercury atom
reappears as a quantum of energy and its wavelength
Davis and Goucher’s method
This experiment is used to subject to a systematic
error on account of the initial velocity of emission of the
electrons from the hot filament.

5. SOMMERFELD RELATIVISTIC ATOM
MODEL
Sommerfeld introduced two main modifications in the
Bohr’s theory:
1) The path of the electron around the nucleus , in general, is an
ellipse with the nucleusat one of the foci.The circular orbits of Bohr are
a special caseof this .
2)The velocity of the electron moving in an ellipticalorbit varies
considerably at different parts of the orbit.This causes relativistic
variation in the mass of the moving electron.Therefore he took into
account the relativistic variation of the massof the electron with
velocity .Hence this is known as THERELATIVISTIC ATOM-MODEL.

6. Vector atom model
1) Bohr’s theory was able to explain the series of the
simplest hydrogen atom.
2) The older theories were inadequate to explain new
discoveries like the zeeman effect and stark effect in
which the spectral lines slit under the influence of
magnetic and electric fields
3) Another drawback of the Bohr model was that it
could not explain how the electrons in an atom
were distributed around the nucleus

7. tHETWO DISTINCT FEATURES OF THE
VECTOR ATOM MODEL ARE:
1) The conception of the spatial
quantisation
2) The spinning electron
hypothesis

8. There are 7 types of quantum
numbers associated with the vector
atom model
• Principal quantum number
• Orbital quantum number
• Spin quantum number
• Total angular quantum number
• Magnetic orbital quantum number
• Magnetic spin quantum number
• Magnetic total angular momentum quantum
number

9. Coupling schemes
1) L-S coupling
In this type all the orbital angular
momentum of the vectors combine to form
a resultant L and independently, all their
spin angular momentum vectors combine
to form a resultant S.
1) j-j coupling
It is employed with the interactions
between spin and orbital vectors of the
each electron

10. Pauli’s exclusion principle
No two electrons in an atom exist in thr
same quantum state.
“No two electrons in an isolated
atom may have the same four quantum
numbers”

11. Periodic classification of elements
The periodic table is an arrangementof differentelements
thatexist the nature,based on theirchemical properties and
theatomic number
Thereare
7 periods and
8 groups
in a periodic table

12. Magnetic dipole moment
They are of 2 types
Namely
• due to orbital motion of the elecctron
• due to spin

13. Stern and Gerlach experiment
This experiment is based on the
behaviour f a magnetic dipole in a non-
uniform magnetic field, the dipole
experiences the torque that tends to
align the dipole parellel to the field