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Physics Notes for Class 12 Chapter 12 Atoms
Dalton’s Atomic Theory
All elements are consists of very small invisible particles, called atoms. Atoms of same element
are exactly same and atoms of different element are different.
Thomson’s Atomic Model
Every atom is uniformly positive charged sphere of radius of the order of 10-10 m, in which
entire mass is uniformly distributed and negative charged electrons are embedded randomly.
The atom as a whole is neutral.
Limitations of Thomson’s Atomic Model
1. It could not explain the origin of spectral series of hydrogen and other atoms.
2. It could not explain large angle scattering of α – particles.
Rutherford’s Atomic Model
On the basis of this experiment, Rutherford made following observations
(i) The entire positive charge and almost entire mass of the atom is concentrated at its centre in
a very tiny region of the order of 10-15 m, called nucleus.
(ii) The negatively charged electrons revolve around the nucleus in different orbits.
(iii) The total positive charge 011 nucleus is equal to the total negative charge on electron.
Therefore atom as a overall is neutral.
(iv) The centripetal force required by electron for revolution is provided by the electrostatic
force of attraction between the electrons and the nucleus.
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Distance of Closest Approach
ro = π εo . 2Ze2 / Ek
where, Ek = kinetic energy of the cc-particle.
Impact Parameter
The perpendicular distance of the velocity vector of a-particle from the central line of the
nucleus, when the particle is far away from the nucleus is called impact parameter.
Impact parameter
where, Z = atomic number of the nucleus, Ek = kinetic energy of the c-particle and θ = angle of
scattering.
Rutherford’s Scattering Formula
where, N(θ) =number of c-particles, Ni = total number of α-particles reach the screen. n =
number of atoms per unit volume in the foil, Z = atoms number, E = kinetic energy of the alpha
particles and t = foil thickness
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Physics Notes for Class 12 Chapter 12 Atoms
Dalton’s Atomic Theory
All elements are consists of very small invisible particles, called atoms. Atoms of same element
are exactly same and atoms of different element are different.
Thomson’s Atomic Model
Every atom is uniformly positive charged sphere of radius of the order of 10-10 m, in which
entire mass is uniformly distributed and negative charged electrons are embedded randomly.
The atom as a whole is neutral.
Limitations of Thomson’s Atomic Model
1. It could not explain the origin of spectral series of hydrogen and other atoms.
2. It could not explain large angle scattering of α – particles.
Rutherford’s Atomic Model
On the basis of this experiment, Rutherford made following observations
(i) The entire positive charge and almost entire mass of the atom is concentrated at its centre in
a very tiny region of the order of 10-15 m, called nucleus.
(ii) The negatively charged electrons revolve around the nucleus in different orbits.
(iii) The total positive charge 011 nucleus is equal to the total negative charge on electron.
Therefore atom as a overall is neutral.
(iv) The centripetal force required by electron for revolution is provided by the electrostatic
force of attraction between the electrons and the nucleus.
www.ncerthelp.com (Visit for all ncert solutions in text and videos, CBSE syllabus, note and many more)
, 2|Page
Distance of Closest Approach
ro = π εo . 2Ze2 / Ek
where, Ek = kinetic energy of the cc-particle.
Impact Parameter
The perpendicular distance of the velocity vector of a-particle from the central line of the
nucleus, when the particle is far away from the nucleus is called impact parameter.
Impact parameter
where, Z = atomic number of the nucleus, Ek = kinetic energy of the c-particle and θ = angle of
scattering.
Rutherford’s Scattering Formula
where, N(θ) =number of c-particles, Ni = total number of α-particles reach the screen. n =
number of atoms per unit volume in the foil, Z = atoms number, E = kinetic energy of the alpha
particles and t = foil thickness
www.ncerthelp.com (Visit for all ncert solutions in text and videos, CBSE syllabus, note and many more)
