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Rutherford’s atom model

Chemistry Formulas

Rutherford’s Nuclear Model of Atom – Discovery of Nucleus

Alpha-particle-scattering experiment:  Rutherford, (in 1909-1911) tested the distribution of charges proposed by Thomson in his model.  He performed some Alpha-particle scattering experiments in which he bombarded thin foils of metals like gold, silver, copper or platinum with a beams of fast moving Alpha-particles (i.e. emitting from Ra or Po).

Rutherford’s Alpha-particles scattering experiment

Rutherford’s atom model


Rutherford’s atom model Observations

The results of Rutherford’s atom model experiment were however, quite unexpected.  He made the following observations:

i) Most of the Alpha-particles passed through the gold foil undeflected.

ii) A small fraction of the Alpha-particles was deflected by small angles.

iii) Very few Alpha-particles (1 in 20,000) were deflected by nearly 180o i.e. bounced back.

Conclusions of Rutherford’s atom model

On the basis of these observations, Rutherford drew the following conclusions regarding the structure of an atom:

i) Most of the space in the atom is empty as most of the Alpha-particles passed through the foil.

ii) A few positively charged alpha particles were deflected through small angle.  These could be deflected only by some positive body present within the atom, concentrated in a very small volume.  This very small portion of the atom was called nucleus by Rutherford.

Rutherford’s Atomic Model

On the basis of Alpha - particle experiment, Rutherford, put forward his nuclear model in 1912.

Main postulates of Rutherford Model are

i) An atom consists of two parts i.e. nucleus and extra nuclear part.

ii) All the mass and entire positive charge of an atom is concentrated in the centre of atom called nucleus.

iii) The positive charge of a nucleus is due to proton, but mass of the nucleus is due to both protons and neutrons.

iv) The total number of protons and neutrons in the nucleus determines the nuclear mass.

v) Most of the space outside the nucleus is empty.

vi) Electrons are distributed in extra nuclear part of atom and revolve around the nucleus at very high speed like planets revolving around the sun, hence electrons are called planetary electrons.

vii) Electrons actually revolve around the nucleus in circular planetary orbits in which the required centripetal force is derived from the electrostatic force of attraction (working) between nucleus and electron.

viii) The dimension of nucleus is of the order of 10-13cm and the dimension of atom is of the order of 10-8 cm. r = R0A1/3, where  R0 (constant) = 1.4 x 10-13cm = 1.4x10-15 m where r = radius of the nucleus, A = mass of the nucleus Thus, Rutherford’s atomic model is comparable with solar system.  So, it is also called as planetary model.

Drawbacks of Rutherford’s atom model

i) It does not obey the Maxwell’s law of electrodynamics – a small charged particle moving under the influence of an attractive force continuously loses energy in the form of electromagnetic radiations.Thus, the electron (a charged body) moving around the nucleus must emit radiations and gradually lose energy.  Due to this loss of energy its motion would slow down and therefore, it would not be able to withstand the attraction of the nucleus. Consequently, the orbit would because smaller and smaller following the spiral path and ultimately the electron would fall into the nucleus. Practically it does not happen.  

Rutherfords atom model

Therefore, Rutherford’s atom model fails to explain the stability of atom(s). 

ii) It could not suggest for the discontinuous nature of spectrum.

iii) It could not explain the emission of various spectral lines during emission spectrum of hydrogen i.e. it fails to explain the atomic spectrum or line spectrum of hydrogen.

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