1) Atoms are the basic building blocks of matter and consist of a nucleus surrounded by electrons. The nucleus contains protons and neutrons, while electrons orbit around the nucleus. 2) Rutherford's gold foil experiment provided evidence that atoms have a small, dense nucleus and that most of an atom's mass and positive charge is concentrated in the nucleus. 3) Bohr's model improved upon earlier models by proposing that electrons orbit in fixed shells and energy levels around the nucleus, explaining the stability of atoms and emission of photons during changes in electron energy levels.

1. ATOMIC
STRUCTURE
Dr.Pradip Patil MD(Radio-diagnosis)
Professor , Dept of Radio-diagnosis
DY Patil Hospital , Kolhapur

2. • Every matter consists of basic entities called
elements.
• Each element is composed of smallest particles called
‘ATOM’.
• Atom- the name is derived from Greek language
Atomos means ‘Not to becut’.
ATOM:

3. Atom and‘DEMOCRITUS’
• DEMOCRITUSwas aGreek
philosopher who began search
for description of matter more
than 2400 years ago(4th century
B.C.)
• Democritus was the personwho
first suggested the existence of
ATOM & coined the name
ÁTOMOS’means Not to be cut
or Indivisible.

4. Atomicstructure
• Atom consists of positively charged
NUCLEUSat the centre andnegatively
charged Electrons revolving aroundit.
• Radius of an atom--10-10m.
• Radius of the nucleus--10-15m.
• Nucleus consists ofProtons and
Neutrons together calledNucleons.
• Most of the massof anAtom is
possessed by Nucleus.

5. Representation of anAtom:
Example:
235U92 X-Uranium
A-235
Z-92

6. Theoriesof Atomicmodels:
JohnDalton
• Matter is made of indivisible atoms,
they areindestructible.
• All atoms of agiven (same) element
are identical in their physical and
chemical properties.
• Atoms of different elements differ in
their physical and chemicalproperties.
• Atoms of different elements combine
in simple whole-numbers ratios to form
Molecules
• Chemical reactions consist ofthe
combination separation or
rearrangement of atoms
• Limitations:It could notexplain
•Why and how do atoms combine together to
form compound atoms(molecules)
•The nature of forces which hold atoms
together in compoundatoms
•Why atoms cannot exist in free state and why
compound atoms can existfreely.

7. Theories of Atomicmodels:
J.J.THOMPSON
Plum pudding model(1904)
• Negative particles are evenly scattered
throughout an atom withapositively charged
massof matter.
• Similar to that of chocolate chip icecream
• Later proved tobe incorrect.
Limitations:
• It could not explain the result of scattering experiment explained by rhe
Rutherford experiment.
• It did not have any experimental evidence in its support .
Thermionic emission, photoelectric emissionandionization were explained on this basis.

8. Theories of Atomicmodels:
RUTHERFORD
• If a thin foil metal is placed in the path of the beam, the image becomes diffuse.
• This due to the scattering of α- particles by the atoms of the foil.
• The particles scattered in various directions were counted by scintillation
counter
• It was found that although most of the particles scattered through angles of
the order of 10or less
• But asmall number say about 1in every 10,000 scattered through 900 or even
1800
• The experiment is known asRutherford's alpha particle scattering experiment.
Rutherford Goldfoil experimentsetup:

9. Resultsof the Rutherfordexperiment
(a) The results that the metal foil
experiment would have yielded if the
plum pudding model had been
correct.
(b) Actualresults

10.  The whole of the positive charge of atom must be concentrated in a very
small space
 Atom is mostly hollowinside
 Since α- particles are positively charged, the part of the atom deflecting
them mustalso be positive
 Most of the massof the atom is concentrated in the nucleus
 In this model, the mass of the atom (leaving the mass of its electrons) and
its whole positive charge are concentrated at the centre of the atom in a
nucleus of radius 10-15m
 The electrons are distributed around the nucleus in a hollow sphere of
radius 10-10m
Conclusionof Rutherfordexperiment

11. Regarding stability of atom
• Electrons revolving around the nucleus have centripetal
acceleration
• According to electrodynamics, accelerated charged
particles radiate energy in the form of electromagnetic
waves
• Hence electromagnetic waves should be continuously
radiated by therevolving electrons
• Due to this continuous loss of energy of the electrons, the
radii of their orbits should be continuously decreasing and
ultimately the electron should fall into the nucleus
• Thus atom cannot remainstable
• Rutherford’s model also failed to explain the Line
spectrum.
Drawback’s of Rutherford’s
Model

12. NielsDavid Bohr
Theories of Atomicmodels:
Bohr’s theory (Postulates)
1.
2.
Fixed circular orbits :The electrons move around the nucleus in
concentric circular orbits .
While revolving in stable orbits, the electrons do not radiate energy
in spite of their acceleration towards the centre of the orbit.
3. Eachof the fixed orbits is associated with a definite amount of
energy called stationary energy.
The energy levels are numbered as1,2, 3, 4… or designated as
K,L,M, N…
4. Jumping of an electron from one energy level to the other (ground state and excited state) .
5.Principle of quantization of angular momentum of the moving electron an electron can move
only in that orbit in which the angular momentum of the electron around the nucleus is an
integral multiple ofh/2π.
• NIELS DAVID BOHR A Danish physicist who developed
Bohr model of atomic structure, in which he introduced the
theory of electrons orbiting around the nucleus.

13. I. No explanation for the spectra of multielectron systems:
Eg:He, Li
I. No explanation of fine spectrum ofatoms:
III. No explanation for Zeeman and Stark effect : effect of
electric and magnetic fields on the spectral atoms.
• When a magnetic field is applied on an atom, its usually
observed spectral lines split. This effect is known as Zeeman’s
effect
• Spectral lines also get split in the presence of electric field. This
effect is known asStarkeffect.
Limitations of Bohr’sPostulates

14.  Isotopes-Elements having same atomic number (protons) , but different
massnumbers (nucleons).
eg: 12C,13Cand14C
are three isotopes of the element carbon with massnumbers 12,13and 14
respectively. Theatomic number of all carbon isotopes is 6.
 Isobars -Elements having same massnumber , but different atomic
numbers.
eg: 40S , 40Cl , 40Ar , 40K, and40Ca
are isobars containing 40 nucleons; however, they contain varying atomic
number.
Classification of atoms:

15.  Isomers -Molecules with the same molecular formula but
different chemical structures. That is, isomers contain the
same number of atoms of each element, but have different
arrangements of their atoms in space.
eg: 131mXe54 isanisomer of 131Xe54
m standsfor meta-stablestate
isomersrepresents identical atomsbut they differ in nuclearenergy states.
 Isotones -Nuclides having very same neutron numberN ,
but different proton number Z.
eg:boron-12 Carbon-13nuclei
both contain 7neutrons

16. NUCLEARSTABILTY:
The Strong Forceis exerted by
anything with massto attract
other massestogetherand works
within avery shortdistance.
Neutrons has no charge,but
have the strong force to bring
other nucleonstogether.
Asageneral rule, anucleus will
need aneutron/proton ratio of 3:2
(or 1.5:1)in order to stay together.
This rule is more precise for larger
nuclei.
Of all known isotopes of natural
elements (about 1500), only 250
of themare stable

17.  .All of these stableisotopeshave an atomic number in between 1and 83.
 The mass of a nucleus will be less than the mass of all of the protons and
neutrons making it up. The difference is called the mass defect, which is
converted into energy if the nucleus is broken up.
 The amount of energy that keeps a nucleons together is called the Binding
Energy. This amount of energy is higher for nuclei that are stable than it
would be for unstable nuclei. (Joules)

18. Binding energy canbe calculatedbythe formula E=mc2
(Einstein,s principle of equivalence of mass& energyrelation)
Where,
c=speed of light ,m=mass &E=Energy
Eg:If a massof 1kg is converted into energy,
m=1kg speed of light is3x108m/s
E=1kg x (3x108m/s)2
E=9 x 1016J
Also,
Mass of electron at rest in terms of energy equivalent is given by,
m=9.1x10-31kgand speed of light is3x108m/s
E=9.1x10-31x (3x108)2
E=8.19x10-14J
E=0.511MeV

19.  Masses of atoms & atomic particles are
conveniently given in terms of amu.
 An amu is defined as1/12of the mass of the
12C6Carbonatom.
1amu= 1.66x10-27 kg
Massexpressedin terms of amuisknown as
atomic massor atomic weight.
Atomic massand Energy Units

20. ChargeandMassof sub-atomicparticles
Particle Chargeof particle Massof theparticle
Proton 1.602176x 10-19 Coulomb 1.00727 amu
Neutron Electrically neutral 1.00866 amu
Electron 1.602176x 10-19 Coulomb 0.00548 amu
• Number of Protons =Number of Neutrons
• 1amu =1/12thof massof an carbon atom.
• 1amu =1.66x10-27 kg

21.  Avagadro's law- Every gram atomic weight of
a substance contains same number of atoms,
thenumber is referred asAvagadro's number
 Value of Avagadro'snumber
6.0221x1023atoms per gram atomicweight
Gramatomic weight

22.  Basic unit of energy is joule (J)
 1Joule is the work done when force of 1Newton is
acting through adistance of1meter.
 Another energy unit in Nuclear physics is electron volt
(eV)
 1eVis defined asthe Kinetic energy acquired by an
electron in passing through apotential difference of
1V.
1eV=1Vx1.602x10-19C
1eV =1.602x 10-19J
Productof Potential difference andChargeof the electron
Atomic energy unit-Joule

23. DISTRIBUTION OFORBITAL
ELECTRONS:
Postulates of Bohrstheory:
a) Electrons can exist in only those orbits for which angular
momentum of electron is an integral multiple of h/2Π
(h= plancksconstant 6.62x10-34 )
b) No energy is gained or lost while an electron remains in any
one ofthe permissible orbits.
Accordingto the model proposedbyNiels
Bohrin 1913
Electrons revolve around thenucleus in
specific orbits.
They are prevented from leaving the
atom by the necessary centripetal force of
attraction between the positively charged
nucleus and negatively chargedelectron.
Bohr’sAtomicmodel

24. Arrangement of electrons inorbitals
Innermost orbit is calledasK-shell.
Followed by orbital’s called L-shell,M-shell and N-shell.
The maximum no. of electrons in an orbital is given by the formula 2n2.
Eg:1)Hydrogen atom has1electron in K-shell
2)Helium atom has2electrons in K-shell
3)Oxygen atom has8 electrons (2 in K-shell,6 inL-shell)

25. NUCLEARFORCES:
There are four types of forces innature
1)Strong nuclear force
2)Electromagnetic force
3)Weak nuclear force
4)Gravitational force
Strong nuclear force -Short range force(~10-15 m )

26. Energylevel diagram of aparticle in anucleus
A)Particle with no
charge
B)Particle with positive
charge
U(r) -Potentialenergy
r -distancefrom centre of nucleus
B-barrier height
R-Nuclear radius

27.  Energylevel diagram of decayof 60Co27nucleus.
60Co27nucleusfirstly emits β-particle with emissionofphotons
Nuclear energy levels:
60Co27
60 Ni28 +e-+ν
Dueto Nuclear transformation anneutron disintegrates in to aproton,an electron
& aneutrino

28. Thank you