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Nov 6, 2018

Rutherford Experiment

Rutherford Experiment:

Rutherford and his students describes some alpha particle scattering experiments. The alpha particles were already established by the group as He⁺² from their behavior in electric and magnetic fields. A beam of alpha particles obtained from spontaneously disintegrating Polonium, were directed on to a very thin foil of Platinum or Gold.
With the help of fluorescent zinc sulphide screen around the Platinum or Gold foil, any deflection of alpha particle was observed. 
Scattering of Alpha Particles from a Metal Foil
Scattering of Alpha Particles from a Metal Foil
The vast majority of the alpha particles passed straight line through the foil. But a very limited few were found to be deflected back from the foil, some even appearing on the side of incidence.
Rutherford concluded that since most of the alpha particles passed straight through there must be a very large volume of empty space in the atom of the platinum or gold.
A very small part of the platinum or gold atom must be responsible for the large scattering of the few alpha particles, central part was called atomic nucleus
Conclusions of Rutherford experiment:
The following Conclusions and an atomic model emerged from the Rutherford's experiment.
1. All the positively charged and almost entirely mass of the atom was concentrated in very small part of the atom and these central core called atomic nucleus.
2.The large deflection of alpha particle from its original path was due to Coulombic repulsion between the Alpha particle and Positive Nucleus of an atom. Simple impact between the two such massive particles can lead to a Scattering of the order of only .
3. An alpha particle suffers little deflection while passing by an electron
4.The Radius of atomic nucleus is ∼ 10⁻¹³ being the same as that of an electron. Since the radius of an atom is ∼ 10⁻⁸ it is obviously that an atom must have a very empty structure. From the above Conclusions an atomic structure is proposed by Rutherford.
Rutherford's Atomic Model:
According to Rutherford's Model the entire mass of the atom was concentrated in a tiny, positively charged nucleus around which the extra nuclear electrons were moving in a circular orbits.   
Rutherford divided the atom into two part,
(i) Centre of Atom or The Nucleus and (ii) Extra nuclear part of atom or The extra nuclear Electrons.
Rutherford's Atomic Model
Rutherford Atomic Model
(i) Centre of Atom or The Nucleus:
Almost the entire mass of the atom is concentrated in a very small, central core called the atomic Nucleus.

Since the extra nuclear electrons contribute negligible to the total mass of the atom ans since the atom is electrically neutral it follows that the nucleus must carry particles which will account both for the mass and positive charge of the atom. 
(ii) The Extra Nuclear Electrons:
A very small positive nucleus was considered surrounded by electrons. Such a system cannot be stable if the election were in rest. Therefore it is proposed that the electron moving in circular orbits around the nucleus so that the Coulombic attraction between the nucleus and the electron was equal to the centrifugal force of attraction.
Rutherford's Model of the Hydrogen Atom
Rutherford's Model of the Hydrogen Atom
Defects of Rutherford's Model:
(i) The Rutherford model is however, not in conformity with the classical model of electromagnetic radiation. A moving charged particle will emit radiation, will then loss kinetic energy and eventually will hit the nucleus.
(ii) If the electrons lose energy continuously, the observed atomic spectra should be continuous, consisting of broad bands merging one into the other.

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