F.Y.B.Sc( As per SPPU Syllabus)

1. PHYSICS PRINCIPLES & APPLICATIONS
CHAPTER 1
PHYSICS OF ATOMS

2. INTRODUCTION
Atom:- Diameter of Atom = 10-10m

3. Mass of proton = 1.6726×10−27 kg
Mass of neutron = 1.6749×10−27 kg
Mass of electron = 9.1056 × 10-31 kg
Mass of electron = 1/1836 times mass of proton
Neutron is Heavier 1.008 times proton
charge of the electron = -1.602 x 10-19 C
charge of the proton = +1.602 x 10-19 C
charge of the neutron = Zero (chargeless)

5. NEUTRAL CARBON ATOM

6. ATOMIC MODELS
THOMSON’S ATOMIC MODEL (1898)

7. Thomson atomic model is compared to
watermelon(PUDDING MODEL).
Where he considered:
Watermelon seeds as negatively charged particles
The red part of the watermelon as positively charged
This model explained the description of an inner structure
of the atom theoretically.

8. POSTULATES OF THOMSON’S ATOMIC MODEL
Postulate 1: An atom consists of a positively charged sphere
with electrons embedded in it.
Postulate 2: An atom as a whole is electrically neutral
because the negative and positive charges are equal in
magnitude.

9. LIMITATIONS OF THOMSON’S ATOMIC MODEL
1. It failed to explain the stability of an atom because his
model of atom failed to explain how a positive charge
holds the negatively charged electrons in an atom.
Therefore, This theory also failed to explain the position
of the nucleus in an atom.
2. Thomson’s model failed to explain the scattering of alpha
particles by thin metal foils.
3. No experimental evidence in its support.

10. APLHA PARTICLE
Alpha particle is doubly charged helium atom who
has lost both of it’s electrons.

11. PROPERTIES OF ALPHA PARTICLE
1. Alpha rays are the positively charged particles.
2. Alpha-particle is highly active and energetic
helium atom that contains two neutrons and
protons.
3. These particles have the minimum penetration
power .
4. These particles have the highest ionization power.

12. RUTHERFORD ALPHA PARTICLE SCATTERING EXPERIMENT
(10-6 m thick)
(ZnS)
(Radium)

13. OBSERVATIONS
Rarely case
1 in 10000
particles .
More than
90o upto
180o
Less than 90o

14. CONCLUSIONS
1) A major fraction of the α-particles bombarded
towards the gold sheet passed through it without any
deflection, and hence most of the space in an atom is
empty.
HYDROGEN ATOM

15. 2) Some of the α-particles were deflected by the
gold sheet by very small angles, and hence
the positive charge in an atom is not uniformly
distributed. The positive charge in an atom is
concentrated in a very small volume.

16. 3) Very few of the α-particles were deflected back,
that is only a few α-particles had nearly 180o angle of
deflection. So the volume occupied by the positively
charged particles in an atom is very small as
compared to the total volume of an atom.
4) Diameter of nucleus is 10-5m.
5) Diameter of atom is 10-10m.

17. RUTHERFORD ATOMIC MODEL
1)The positively charged particles and most of the mass of
an atom was concentrated in an extremely small volume.
He called this region of the atom as a nucleus.
2)Rutherford model proposed that the negatively charged
electrons surround the nucleus of an atom. He also claimed
that the electrons surrounding the nucleus revolve around
it with very high speed in circular paths. He named these
circular paths as orbits.

18. 3)Electrons being negatively charged and
nucleus being a densely concentrated mass of
positively charged particles are held together
by a strong electrostatic force of attraction.

19. DRAW BACKS OF RUTHERFORD MODEL
1)According to Maxwell, accelerated charged particles
emit electromagnetic radiations and hence an
electron revolving around the nucleus should emit
electromagnetic radiation.
This causes continuous loss in energy of revolving
electron, which would come at the cost of shrinking
of orbits. Ultimately the electrons would collapse in
the nucleus.

20. Emission of Energy

21. 2)Revolving electron must emit continuous
radiations of all increasing frequencies.
But it is observed that elements emit spectral lines
of fixed frequencies & not all the frequencies.
3) No mention of distribution of electrons.

22. Bohr model of the atom was proposed by Neil Bohr in
1915.
Bohr modified this atomic structure model by explaining
that electrons move in fixed orbital’s (shells).
He also explained that each orbit (shell) has a fixed
energy level, & electron is allowed to occupy only certain
orbits of specific energies
BOHR ATOMIC MODEL

23. ATOMIC SPECTRA
In 1814, Joseph Fraunhofer made detailed examination of
solar spectrum.
There are 700 lines in solar spectrum called Fraunhofer
lines. These lines are dark in spectrum.
In 1985, J.J. Balmer published a formula for wavelengths of
hydrogen atom in visible region.
In 1906 Lyman works on ultra violet spectra, called Lyman
series.
Paschen investigated infrared series, called Paschen Series.
Pfund & Brackett investigated infrared series .

24. EXPERIMENTAL SETUP TO STUDY ATOMIC SPECTRA

25. WAVE NUMBERS OF SPECTRAL LINES
Where,
n1 – Final state
n2 – Initial State
R – Rydberg constant = 10.9679 μ/m

26. EMISSION OF SPECTRA
The line emission line spectrum results from
electrons dropping from higher energy level to lower
energy levels.
Each time an electron drops, a proton of light is
released whose energy correspond to the difference
in energy between the two levels(E = hϑ)

27. EMISSION SPECTRUM
ABSORPTION SPECTRA
CONTINUOUS SPECTRA

28. BOHR POSTULATES
Postulate 1:-
The negative electron moves around the positive
nucleus (proton) in a circular orbit. All electron orbits
are centered at the nucleus. The Centripetal Force is
responsible for circular motion of electron.

29. Let ,
M - mass of electron
Z - Atomic number
e – Charge on electron
v – velocity of electron
r – radius of orbit
Ze – Charge on nucleus
εo – Permittivity of free Space = 8.85*10-12 F/m

30. Coulomb's Force between electron & nucleus,
Centripetal Force is,
According to Bohr’s 1st postulate,

31. Postulate 2:-
The motion of an electron in a circular orbit is
restricted in such a manner that its angular
momentum is an integral multiple of h/2π.
mvr = nh/2π,
Where;
m = mass of the electron
v = velocity of the electron
r = radius of the orbit
n = an integer called principal quantum number
of the electron.

32. Postulate 3 :-
An electron may jump spontaneously from one orbit
(energy level E1) to the other orbit (energy level E2)
(E2 > E1); then the energy change AE in the electron
jump is given by Planck’s equation
∆E = E2-E1 = hϑ
Where,
h = Planck’s constant.
ϑ = frequency of light emitted.

33. STRENGTHS OF BOHR MODEL
1)He explain the size of hydrogen atom. The radius of hydrogen
atom is calculated by using this model.
2) An ionization potential value of hydrogen atom is given by
bohr model.
3)He explains the formation of spectral lines in hydrogen
atom, also give strong scientific basis for observed emission &
absorption spectral lines.
4)He gave explanation for structure of electron cloud.

34. 1) Bohr's model of an atom could not account for the finer
details of the hydrogen spectrum.
2) He could not explain line spectra of atoms containing more
than one electron.
3) He failed to explain the effect of magnetic field on the
spectra of atoms or ions, that is splitting of spectral lines
further in presence of magnetic field called as
Zeeman effect.
DRAWBACKS OF BOHR MODEL

35. 4) He could not explain the effect of electric field on the
spectra of atoms (known as Stark effect)
5) He could not provide clue to explain the shapes of
molecules arising out of the directional bonding
between atoms.
6) He does not explain why some spectral lines are
brighter than others.

36. FRANK HERTZ EXPERIMENT
Mercury Atoms Electron

37. ELECTRIC CURRENT VS VOLTAGE

38. When the accelerating voltage reaches 4.9 V (lowest energy
required by the mercury atom for excitation), the current
drops sharply. This drop is due to inelastic collisions between
the accelerated electrons and electrons in the mercury
atoms, will not reach the collector.
At 9.8 V, each electron gets sufficient energy to participate in
two inelastic collisions. They excite two mercury atoms, and
no energy will be left. This process will repeat, for each
interval of 4.9 V.
Collected current drops at multiples of 4.9 V.
CONCLUSION

39. Two types of collision are there ,
1) Elastic collision 2) Inelastic collision
Elastic Collision:- When Electron collide with mercury
atom.
Inelastic Collision:- When Electron collide with mercury
atom and lift the electron in
mercury atom from lower orbit to
higher orbit.

41. Frank and Hertz measure the wavelength of
emitted radiation by spectroscopic techniques.
They get value = 2537 Ao
Photon energy E = h ϑ
But, c = λ ϑ
E = hc/λ = (6.6*10-34 *3*108)/ 2537*10-10
E = 7.83*10-19 J
E = 4.9 eV
This shows experimental values are true.
In mercury atom energy levels are quiet sharp &
discrete & cannot exists intermediate energy state.