The α -scattering from metal foils. The particles are produced by the radioactive source. Since lead absorbs α -particles a lead plate with a hole is used to obtain a beam of α-particles.
The particles scattered from the metal foil strike the fluorescent (zinc sulphide) screen and produce tiny flashes. A movable microscope is used to view flashes.
i. Most of the α-particles passed through the metal foil without any change in their path
i.e. they remained undeflected.
ii. Some of the α -particles
iii. Only a few of them (1 in 10,000) were actually deflected by as much as 90°, or even
larger angles. One in 20,000 particles returned back suffering a deflection of 180°.
Figure 5.
(a)
(b)
Explanation.
The results of the scattering experiment could not be explained by the Thomson's atomic theory.
Calculations showed that a charge spread over a sphere of radius 10^(-8) cm could deflect α-particles only through small angles. The deflection of α-particles only through large angles as observed would be possible if the positive charge in the atom is spread over a sphere Of radius measuring about 10^(-13) cm. The resulted scattering of α-particles could therefore by no means be explained by Thomson's atomic model.
RUTHERFORD NUCLEAR MODEL OF THE ATOM
On the basis of α-particles scattering experiment, Rutherford put forward in 1912, his nuclei model of the atom.
According to his assumptions that.
i. An atom consists of a positively charged nucleus surrounded by a system of negatively charged particles called electrons. Electrons revolve around the nucleus, the positive charges, i.e. protons are present in it.
ii. Electrons and the nucleus are held together by a strong electrostatic force of attraction.
iii. The effective volume of the nucleus is extremely small as compared to the effective volume of the atom.
From the experiments, it was found that, the Approximate radius of the Nucleus of an atom measures about 10^(-14) to 10^(-15) m {10^(-12) to 10^(-13)cm}, The approximate radius of sphere of electrons (or the radius of an atom) is 10^(-10)m {10^(-8)cm}. Since the volume varies as r³, hence the volume occupied by the nucleus is about 10¹² times the volume of the atom.
iv. Almost the entire mass of the atom is concentrated in the nucleus.
v. The positive charge on the nuclei of different elements are always integral multiples of
the electron charge, but opposite in sign. Since, each atom is electrically neutral, hence in
an atom, the number of positive charges on the nucleus of an atom is equal to the number of
electrons in it.
The Rutherford model of an atom is shown in the figure below.
Figure 6.
According to this model an atom consists of positively charged nucleus which is surrounded by a system of electrons. The electrons and a
nucleus are held together by electrostatic forces. Because of the electrostatic force of attraction between
the nucleus and the electrons, the electrons should ultimately fall into the nucleus. But, it does not happen
In order to explain why electrons do not fall into the nucleus, Rutherford postulated that the electrons are not stationary, but are revolving about the nucleus in orbits. But, this
explanation did not solve the problem completely.
The physicists had observed that a revolving electric charge must lose energy by emitting radiation. Thus, an electron revolving around a nucleus in an orbit should emit radiation and lose energy. By so doing, the continuous energy loss should slow down electrons. As a result, the electron will not afford to stand the nuclear attraction and gradually move towards the nucleus. The electron should, therefore, follows a spiral path which ultimately fall into the nucleus within 10^(-8)s.
The particles scattered from the metal foil strike the fluorescent (zinc sulphide) screen and produce tiny flashes. A movable microscope is used to view flashes.
i. Most of the α-particles passed through the metal foil without any change in their path
i.e. they remained undeflected.
ii. Some of the α -particles
iii. Only a few of them (1 in 10,000) were actually deflected by as much as 90°, or even
larger angles. One in 20,000 particles returned back suffering a deflection of 180°.
Figure 5.
Explanation.
The results of the scattering experiment could not be explained by the Thomson's atomic theory.
Calculations showed that a charge spread over a sphere of radius 10^(-8) cm could deflect α-particles only through small angles. The deflection of α-particles only through large angles as observed would be possible if the positive charge in the atom is spread over a sphere Of radius measuring about 10^(-13) cm. The resulted scattering of α-particles could therefore by no means be explained by Thomson's atomic model.
RUTHERFORD NUCLEAR MODEL OF THE ATOM
On the basis of α-particles scattering experiment, Rutherford put forward in 1912, his nuclei model of the atom.
According to his assumptions that.
i. An atom consists of a positively charged nucleus surrounded by a system of negatively charged particles called electrons. Electrons revolve around the nucleus, the positive charges, i.e. protons are present in it.
ii. Electrons and the nucleus are held together by a strong electrostatic force of attraction.
iii. The effective volume of the nucleus is extremely small as compared to the effective volume of the atom.
From the experiments, it was found that, the Approximate radius of the Nucleus of an atom measures about 10^(-14) to 10^(-15) m {10^(-12) to 10^(-13)cm}, The approximate radius of sphere of electrons (or the radius of an atom) is 10^(-10)m {10^(-8)cm}. Since the volume varies as r³, hence the volume occupied by the nucleus is about 10¹² times the volume of the atom.
iv. Almost the entire mass of the atom is concentrated in the nucleus.
v. The positive charge on the nuclei of different elements are always integral multiples of
the electron charge, but opposite in sign. Since, each atom is electrically neutral, hence in
an atom, the number of positive charges on the nucleus of an atom is equal to the number of
electrons in it.
The Rutherford model of an atom is shown in the figure below.
According to this model an atom consists of positively charged nucleus which is surrounded by a system of electrons. The electrons and a
nucleus are held together by electrostatic forces. Because of the electrostatic force of attraction between
the nucleus and the electrons, the electrons should ultimately fall into the nucleus. But, it does not happen
In order to explain why electrons do not fall into the nucleus, Rutherford postulated that the electrons are not stationary, but are revolving about the nucleus in orbits. But, this
explanation did not solve the problem completely.
The physicists had observed that a revolving electric charge must lose energy by emitting radiation. Thus, an electron revolving around a nucleus in an orbit should emit radiation and lose energy. By so doing, the continuous energy loss should slow down electrons. As a result, the electron will not afford to stand the nuclear attraction and gradually move towards the nucleus. The electron should, therefore, follows a spiral path which ultimately fall into the nucleus within 10^(-8)s.
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