The document provides a detailed overview of the historical development of atomic theory, beginning with early philosophers like Democritus and moving through significant figures such as Dalton, Thomson, Rutherford, Chadwick, Bohr, and Schrödinger. It discusses models of atomic structure including the plum-pudding model, nuclear model, and electron cloud model, as well as fundamental concepts like isotopes, binding energy, and nuclear stability. Additionally, it explains processes like nuclear fission and fusion, emphasizing the relationship between neutrons and protons in maintaining nuclear stability.

1. STRUCTURE OF ATOM
Made by :Dr. Isha jaiswal
Moderator :Dr. S.P.MISHRA

3. •Changes in view of “aToM”

4. •Democritus (400 B.C.)
• Proposed that matter was
composed of tiny indivisible
particles
• Philospher; Not based on
experimental data.
• Greek: atomos

5. •John Dalton (1807)
• British Schoolteacher
• Dalton’s postulate
• Proposed Ball Model
• atom is a
uniform,
solid sphere

6. •J. J. Thomson
• Cathode Ray Tube
Experiments
• beam of negative
particles
• Discovered Electrons
• negative particles within
the atom
• Plum-pudding Model

7. •Thomson’s Cathode Ray Experiment
Stream of electrons is attracted to positively charged plate here.
"

8. •J. J. Thomson
Plum-pudding Model
• positive sphere
(pudding) with
negative electrons
(plums) dispersed
throughout

9. •Ernest Rutherford
Student of J.J THOMSON
Nobel prize winner.
Rutherford performed the famous
‘GOLD FOIL EXPERIMENT’.
Discovered NUCLEUS :dense, positive
charge in the center of the atom.
Proposed NUCLEAR MODEL

10. •Gold Foil Experiment

11. •Gold Foil Experiment Cont.

12. • Nuclear Model
• dense, positive nucleus surrounded
by negative electrons

13. •James Chadwick (1932)
• Discovered neutrons
• neutral particles in the
nucleus of an atom

14. •James Chadwick (1932)
Neutron Model
• revision of Rutherford’s Nuclear Model

15. •Niels Bohr • Father of QUANTUM
PHYSICS
• Revised RUTHERFORD’S
model .eletron is
accelerating charge.
• Energy Levels
• electrons can only exist
in specific energy states

16. • Bright-Line Spectrum :
• tried to explain presence
of specific colors in
hydrogen’s spectrum
• Planetary Model
• electrons move in circular
orbits within specific
energy levels
Bright-line spectrum

17. •Erwin Schrödinger :
Electron Cloud Model (orbital)
• dots represent probability of finding an e-
not actual electrons

18. ELECTRON CLOUD Model
This model is based
upon Bohr’s model,
except that electrons
orbit the nucleus in
random patterns. The
region where these
particles are found is
referred to as the
electron cloud.
Electron Clouds
Nucleus

19. • The Nucleus and Structure of the
Atom
• Atoms are made of three kinds of
particles: electrons, protons, and
neutrons.

20. • The structure of the atom
• The protons &
neutrons are present
in nucleus
• Electrons are
outside the nucleus
in the electron
cloud.

21. • The representation of atom

22. Nomenclature for Elements
"X" = Element Symbol
"Z" = ATOMIC NUMBER: no.of Protons
Each eleMenT has a unique "Z”
"N” = no.of Neutrons
"A" = ATOMIC MASS:no.of neutron +proton ( A = Z + N)
ISOTOPE: Atoms of same elements with same
atomic no. but different mass no.
Isobar: ATOMS OF DIFFERENT ELEMENT HAVING SAME
MASS NO. BUT DIFFERENT ATOMIC NO.
X
A
Z

23. ATOMIC &NUCLEAR
STABILITY: the
forces of NATURE.

24. • The strong nuclear force attracts
neutrons and protons to each other,
otherwise the positively charged protons
would repel each other.

25. • Electrons are bound to the
nucleus by
electromagnetic forces.
• The force is the attraction
between protons
(positive) and electrons
(negative).
 The momentum of the electron causes it to move
around the nucleus rather than falling straight in.

26. 26
• As a general rule, a
nucleus will need a
neutron/proton ratio
of 3:2 (or 1.5:1) in
order to stay
together.

27. 27
As atomic mass increases,
the neutron to proton ratio for
stable nuclei increases
because proton-proton
repulsion becomes significant!!!
Nuclear forces arise form
neutrons, so the neutron to
proton ratio must increase for
heavier elements.
Belt of Stability
Proton number, Z
Neutronnumber,N=A-Z
N = Z
For helium He- 4 the N:P ratio is 1 : 1
For uranium U- 238 the ratio is 1 : 1.6

28. • If an isotope has too many (or too few)
neutrons, the nucleus eventually breaks up
and we say the atom is radioactive.
• In a stable isotope the nucleus stays
together.

29. 29
• The amount of
energy that keeps a
nucleus together is
called the Binding
Energy.
• This amount of
energy is higher for
nuclei that are stable
than it would be for
unstable nuclei.

30. 30
Mass Defect
The difference
between the mass
of the atom and
the sum of the
masses of its parts
is called the mass
defect (Dm).
measurements show that the mass of a particular atom is always
slightly less than the sum of the masses of the individual protons,
neutrons and electrons of which the atom consists.
e.g. a helium nucleus consists of 2 protons and 2 neutrons
2 protons & 2 neutrons Helium atom

31. 31
Mass defect can be converted into equivalent energy called as binding energy.
Using Einstein’s E = mc2, this is equivalent to a loss of energy
This figure is the BINDING ENERGY .
THE BINDING ENERGY of a nucleus is defined as the energy which must be input
to separate all of its protons and neutrons.

32. 32
Binding Energies are usually expressed in MeV
1 amu = 931.3 MeV
To compare the stabilities of different nuclei,
Binding Energies PER NUCLEON in the nucleus are compared.
The higher the binding energy per nucleon,
the greater the stability of the nucleus

33. 33
NUCLEON NUMBER
BINDING ENERGY
Per NUCLEON
2H
238U
56Fe
8.8 MeV
Iron is the most stable nucleus

34. 34
FISSION
IRON
Heavy nuclei may increase their stability by Nuclear Fission
Light nuclei may increase their stability by Nuclear Fusion
FUSION

35. 35
Nuclear Fission is the fragmentation of heavy nuclei to form lighter, more stable ones.
The Fission of U - 235
U235
92
energyofMeVnKrBanU 18031
0
94
36
139
56
1
0
235
92 
This is only one of several fissures that are possible.
On average 2.5 neutrons are released

36. 36
Critical mass :is the mass required for the
chain reaction to become self-sustaining.
neutron
Some neutrons may :
Cause
more
fission
Get
lost
Be
absorbed
by an atom
lost
For a chain reaction to be self sustaining,
every fission must produce at least one more
neutron that will initiate further fissions.

37. Every good conversation starts with good
listening.
Thankyou verymuch for ur patient listening.
Special thanxs to S.PMISHRA SIR for his
guidance and support.