Download SSPD Chapter 1_Part 5_Story of Atom-Solar

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SSPD Chapter 1_Part 5_Story of
Atom-Solar System Model of Atom
∗
as proposed by Rutherford.
Bijay_Kumar Sharma
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Creative Commons Attribution License 3.0†
Abstract
SSPD_Chapter 1_Part 5 describes the historic experiment of "Bombarding the Gold Foil with Alpha
particles". This experiment established that most of the atom is empty and all the positive part is
concentrated in the nucleus which contains practically all the mass also and that negatively charged
electron are orbiting the nucleus just as Earth is orbiting the Sun.
1.6.2. SOLAR SYSTEM MODEL OF ATOM AS PROPOSED BY ERNEST RUTHERFORD.
From 1906 to 1908 by particle collision and particle scattering, Earnest Rutherford proved that nearly
99.9% of the mass is concentrated in the nucleus of the Atom and that the diameter of the nucleus is 4
orders of magnitude smaller than the diameter of the Atom. The diameter of the Atom is of the order of
Angstrom(10-10 m) whereas the nucleus is of the order of 10 Fermi Unit where 1 Fermi Unitis 10-15 m. Since
Atoms are electrically neutral hence nucleus contains as many positive charges as are the electrons orbiting
the nucleus.
From this Model, most of the Atom is vacuum. Just as the Planets are orbiting the Sun, in an analogous
manner electrons are orbiting the nucleus. Furthermore the nucleus contains as many positively charged
protons as the number of electrons are orbiting the nucleus.
In the particle scattering experiment a Gold or Platinum thin foil was bombarded with Alpha particles.
Most of the particles passed through the foil without experiencing any deection. It appeared as if the
interatomic and most of the intra-atomic space in the foil was empty space.
Though most of the Alpha particles passed through unhindered there was a minority group of Alpha
particles which did experience small deection and there was a rare few which were completely reversed in
their track. The reversal in their track was tantamount to a deection of 180 ◦ as shown in Fig(10).
∗ Version
1.1: Jan 17, 2010 10:49 pm -0600
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Figure 1
Figure 1.10. The scattering phenomena of Alpha Particles by a thin Gold Foil.
Since the electrons are 1838th part of proton by mass therefore the orbiting electrons have no inuence
on incident Alpha particles which ,as will be seen ,are the nuclei of Helium Atoms. But if the incident Alpha
particle experiences a direct collision with the nucleus, the former is reversed in its track. This clearly proves
that Alpha particles are of comparable mass as that of the nucleus.
Here it will be appropriate to mention that Alpha , Beta and Gamma Rays accompany Radioactivity.
Gamma Rays are very short energetic Electromagnetic Waves of wavelength (1/100)Å, Beta Rays are electron
beam whereas Alpha Rays are the nuclei of He atoms with two protons and two neutrons.
On the basis of α particles scattering by gold foil, Rutherford proposed that most of the atom is empty
space with 99.9% of the total mass concentrated at the center called nucleus and electrons are orbiting
around the nucleus. The negative charge of the orbiting electrons is exactly balanced by the positive charge
of the nucleus.
In 1908 Earnest Rutherford was awarded the Nobel Prize in Physics for unraveling the planetary model
of the Atom.
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