This document provides a history of atomic structure models from ancient Greek philosophers to modern physics. It summarizes Democritus' idea of indivisible atoms, Dalton's billiard ball model of atoms, Thomson's plum pudding model, and Rutherford's nuclear model based on his gold foil experiment. It then describes Bohr's planetary model with quantized electron orbits and energy levels. Later models like Sommerfeld's addressed limitations like explaining hydrogen's fine structure. The document traces how atomic structure models evolved as new experimental evidence emerged.

1. ATOMIC STRUCTURE
Submitted by: Aditya Dey
Roll no-1 ,4th sem
Paper-401
Submitted to : Dr. P. K. SUKLA
Assam university,silchar

2. CONTENTS :
• Introduction
• The Greek model
• The Dalton Model: Billiard Ball Model
• What did Dalton base his theory on?
• Limitation of dalton's model
• J.J. Thomson Model- Plum-pudding Model
• Problems with Thomson’s Model
• Ernest Rutherford’s Model
• Limitations of Rutherford's Model
• The Bohr Model: The Planet Model
• Bohr’s Postulates
• Quantisation of velocity and radius and energy
• Sommerfeld’s relativistic atom model
• Conclusion
• References

3. •INTRODUCTION:
•Atoms are so small that, even today, direct visual
inspection is all but impossible. Our model of the atom is
based on indirect experimental data. Because of this, our
model of the atom changes as our experimental ability
improves

4. l The Greek Model
•Democritus - A Greek Philosopher around
the year 400 BC.
Democritus concluded that matter could not
l
be divided into smaller and smaller pieces
forever.
Eventually, the smallest piece of matter
l
would be found. He used the word
"Atomos“ meaning indivisible to describe
the smallest possible piece of matter.

5. The Dalton Model: Billiard Ball
l
Model
•John Dalton - The English chemist that
proposed
• first Atomic Theory in 1803.
•All elements are composed of indivisible
particles.
•Atoms of the same element are exactly alike.
•Atoms of different elements are different.
•Compounds are formed by joining atoms of
two or more elements

6. l What did Dalton base his theory on?
–Law of Conservation of Mass (Antoine
Lavosier, 1789)
•Atoms are neither created or destroyed
(under normal chemical reactions)
–Law of Definite Proportion, (Proust's Law)
•Atom ratio is fixed, so mass must be
constant.
–Law of Multiple Proportions (Dalton)
•Applies where two elements, A and B, form
more than one compound.

7. LIMITATION OF DALTON'S MODEL:
lMost of Dalton’s theory is accepted today
•However, We now know that atoms ARE divisible
•Atoms contain sub-atomic particles
•ELECTRONS
•PROTONS
lNEUTRONS

8. J.J. Thomson Model- Plum-pudding Model:
lJ. J. Thomson 1856 – 1940 - The English scientist who
ldiscovered Electrons in 1887.
Thomson determined the charge
of an electron to be negative in his
cathode ray experiment
Thompson initially called electrons
CORPUSLES.

9. Thomson’s Experiment
•In 1897
•Ran electric current through tube
•Positively Charged metal plates bent the light
beam
•Discovery of the small, negatively-charged
electron
•Atoms are made of smaller parts

10. Thomson’s Model of the Atom
•Negatively charged electrons
scattered through positive mass
•“Plum pudding” model
–Plums are negative
–Pudding is positive

11. Problems with Thomson’s Model
l
•How does the atom emit radiation?
•This model soon came into conflict with
experiments by Rutherford

12. Ernest Rutherford’s Model :
•Ernest Rutherford - The British
physicist who, in 1908, proved the atom
had a small, dense, positively charged
Nucleus.
Rutherford's model proposed that an
atom is mostly empty space. There is a
small, positive nucleus with the negative
electrons scattered around the outside
edge.

13. Rutherford’s Experiment
•1911
•Shot positive alpha particles
•Expected them to pass
through but some bounced
back
•Atoms are mostly empty
•Nucleus is dense and positive

14. Rutherford’s Model of the Atom
•Rutherford concluded that the
atom is mostly empty space
•However, the atom contains a
nucleus where the mass and
positive charge of the atom are
concentrated
•We now know that the nucleus
contains PROTONS and
NEUTRONS
•Electrons ORBIT the nucleus but
are not part of it

15. Limitations of Rutherford's Model :
According to Rutherford's model electron rotates
with uniform velocity around Nucleus.
But,unfortunately , uniform rotation is an accelerated
motion and according to classical E.M theory , an
accelerated charge emits radiation and the
consequent loss of energy would make the electron
spiral into the nucleus. And the time taken for the
process is estimated to be about
10^(-8) second. Thus
Rutherford atom model
can not be stable.

16. l The Bohr Model: The Planet Model
Niels Bohr - The Danish scientist who, in
1913, proposed the Planetary Model of the
atom.
•Electrons move in definite orbits around the
nucleus, like planets moving around the
nucleus. Bohr proposed that each electron
moves in a specific energy level.

17. Bohr’s Postulates
l
•Bohr started from the assumption
that the electron moves in circular
orbits around the proton under the
influence of the attractive electric
field.
•Postulate 1: Only certain orbits are stable.
These are stationary or more precisely quasi-
stationary states. An electron does not emit
EM radiation when in one of these states
(orbits)

18. •Bohr’s Postulates
•Postulate 2. The electron can only have an
orbit for which the angular momentum of the
electron, L, takes on discrete values (the orbits
are quantized):
L = me vr = n
h
=
2π
•Orbits characterized by angular momentum since this
depends on both the distance of the electron from nucleus
and its velocity

19. •Bohr’s Postulates
•Postulate 3. If the electron is initially in an
allowed orbit (stationary state), i, having the
energy, Ei, goes into another allowed
orbit, f, having energy, Ef (< Ei), EM radiation is
emitted, with energy and frequency,
hν = Ei Ef
Ei Ef
ν=
h

20. Quantisation of velocity and
radius and energy :
Combining the quantisation of angular momentum and
the equation of mechanical stability we arrive at the
result that:
the allowed radius and velocity at a given orbit are also
quantised:
n2 2 1 Ze 2
rn 4 0 vn
me Ze 2 4 0 n

21. Energies in the hydrogen-like
atom
Potential energy of the electron at a distance r from the nucleus is, as
we learned from standard electrostatics, ZCT 102, form 6, matriculation
etc. is simply
Ze 2 Ze 2
V 2
dr
r 4 0r 4 0r
• -ve means that the EM force is attractive
-e
+Ze Fe

22. Kinetic energy in the hydrogen-like
atom
According to definition, the KE of the electron is
me v 2 Ze2
K
2 8 0r
me v 2 Ze2
The last step follows from the equation r 4 0r2
• Adding up KE + V, we obtain the total
mechanical energy of the atom:
Ze2 Ze2 Ze2 1 Ze2 me Ze2
E K V
8 0r 4 0r 8 0 r 8 0 4 0 n 2 2
me Z 2e 4 1
2 2
En
4 0 2 2 n

23. 1 1 1
RH
n2
f ni2
For Lyman series, n f 1, ni 2,3, 4,...
For Balmer series, n f 2, ni 3, 4,5...
For Paschen series, n f 3, ni 4,5, 6...
For Brackett series, n f 4, ni 5, 6, 7...
For Pfund series, n f 5, ni 6, 7,8...

24. LIMITATION OF BOHR’S MODEL :
1. In Bohr’s model the quantum idea of
stationary orbits is mixed up with the classical
idea of coulomb force.
2. The assumption of only circular orbit is utterly
unjustified.
3. It can only explain the line of the hydrogen
and hydrogen like atoms.
4. It can not explain the fine structure of
hydrogen atom.
5. It can not make any calculation about the
transition or the selection rules which apply to
them.

25. SOMMERFELD’S RELATIVISTIC ATOM MODEL :
To explain the fine structure of spectral lines Sommerfeld
introduced two modification in Bohr’s theory,
1. According to Sommerfeld the path of an electron
around the nucleus , in general ,is an ellipse with the
nucleus at one of the foci. The circular orbit is the
special case of the ellipse.
2. The velocity of the electron moving in an elliptical orbit
varies 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 mass of the electron with velocity. Hence this is
known as relativistic atom model.

28. REFERENCES :
1. MODERN ATOMIC AND NUCLEAR PHYSICS- A. B.
GUPTA
2. ATOMIC AND MOLECULAR SPECTRA-LASER –RAJ
KUMAR