CBSE Class-12 Physics Quick Revision Notes
Chapter-12: Atoms
• Thomson’s Model of an Atom:
An atom consists of positively charged matter in which the negatively charged electrons
are uniformly embedded like plums in a pudding. This model could not explain
scattering of alpha-particles through thin foils and hence discarded.
• Rutherford’s Model of an Atom:
Geiger and Marsden in their experiment on scattering of alpha-particles found that most
of the alpha-particles passed undeviated through thin foils but some of them were
scattered through very large angles.
From the results of these experiments, Rutherford proposed the following model of an
atom:
a) An atom consists of a small and massive central core in which the entire positive
charge and almost the whole mass of the atom are concentrated. This core is called
the nucleus.
b) The nucleus occupies a very small space as compared to the size of the atom.
c) The atom is surrounded by a suitable number of electros so that their total negative
charge is equal to the total positive charge on the nucleus and the atom as a whole is
electrically neutral.
d) The electrons revolve around the nucleus in various orbits just as planets revolve
around the sun.
e) The centripetal force required for their revolution is provided by the electrostatic
attraction between the electrons and the nucleus.
• Draw-back of Rutherford Model:
This model could not explain in stability of the atom because according to classical
electromagnetic theory the electron revolving around the nucleus must continuously
radiate energy revolving around the nucleus must continuously radiate energy in the
form of electromagnetic radiate energy in the form of electromagnetic radiation and
hence it should fall into the nucleus.
• Distance of Closest Approach:
When an alpha-particle of mass m and velocity v moves directly towards a nucleus of
atomic number Z, its initial energy E, which is just the kinetic energy K gets completely
converted into potential energy U at stopping point. This stopping point happens to be at
a distance of closest approach d from the nucleus.
1 2 1 2eZe 2 Ze 2 2 Ze2
E = mv = = d=
2 4πε 0 d 4πε 0 d 4πε 0 k
Hence,
2 Ze 2
d=
4πε 0 K
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Chapter-12: Atoms
• Thomson’s Model of an Atom:
An atom consists of positively charged matter in which the negatively charged electrons
are uniformly embedded like plums in a pudding. This model could not explain
scattering of alpha-particles through thin foils and hence discarded.
• Rutherford’s Model of an Atom:
Geiger and Marsden in their experiment on scattering of alpha-particles found that most
of the alpha-particles passed undeviated through thin foils but some of them were
scattered through very large angles.
From the results of these experiments, Rutherford proposed the following model of an
atom:
a) An atom consists of a small and massive central core in which the entire positive
charge and almost the whole mass of the atom are concentrated. This core is called
the nucleus.
b) The nucleus occupies a very small space as compared to the size of the atom.
c) The atom is surrounded by a suitable number of electros so that their total negative
charge is equal to the total positive charge on the nucleus and the atom as a whole is
electrically neutral.
d) The electrons revolve around the nucleus in various orbits just as planets revolve
around the sun.
e) The centripetal force required for their revolution is provided by the electrostatic
attraction between the electrons and the nucleus.
• Draw-back of Rutherford Model:
This model could not explain in stability of the atom because according to classical
electromagnetic theory the electron revolving around the nucleus must continuously
radiate energy revolving around the nucleus must continuously radiate energy in the
form of electromagnetic radiate energy in the form of electromagnetic radiation and
hence it should fall into the nucleus.
• Distance of Closest Approach:
When an alpha-particle of mass m and velocity v moves directly towards a nucleus of
atomic number Z, its initial energy E, which is just the kinetic energy K gets completely
converted into potential energy U at stopping point. This stopping point happens to be at
a distance of closest approach d from the nucleus.
1 2 1 2eZe 2 Ze 2 2 Ze2
E = mv = = d=
2 4πε 0 d 4πε 0 d 4πε 0 k
Hence,
2 Ze 2
d=
4πε 0 K
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