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Discovery of Proton
DISCOVERY OF PROTON:
POSITIVE RAYS OR CANAL RAYS
Atoms are electrically neutral. Hence after the discovery of the negatively charged constituent (electron) of an atom, attempts were made to discover the positively charged counterpart of electrons. By using a discharge tube containing a perforated cathode. Goldstein (1886) found that some rays passed through these holes in a direction opposite to that of the cathode rays.
Alt text: POSITIVE RAYS OR CANAL RAYS
These are called the positive rays or canal rays. J.J. Thomson (1910) measured their charge by mass ratio from which he was able to deduce that these contain positive ions. Their properties are:
- They are positively charged.
- The positive charge is either equal to or whole number multiple of the charge on an electron.
- When hydrogen gas was filled in the discharge tube the positive charge on the positive rays was equal to the negative charge on an electron, and the mass was less than the hydrogen atom.
- Unlike cathode rays the properties of positive rays are characteristics of the gas in the tube.
- The deflection of positive rays under the influence of an electric or magnetic field is smaller than that of the cathode rays for the same strength of field. This shows that the positive rays have a greater mass than that of electrons.
- The mass of the positive rays depends on the atomic weights or molecular weights of the gases in the discharge tube. The charge/mass ratio also varies because the change in positive charge on the rays. It may be either equal to or integral multiple of the charge on an electron.
- The lightest of all particles identified in positive rays from different elements was one with a mass very slightly less than that of hydrogen atom (or nearly equal to H-atom). The lightest positively charged particle is called a proton (P or P+). Positive rays are atomic or molecular resides from which some electrons have been removed. The removed electrons constitute the cathode rays and the positive residues form the positive or canal rays.
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Positive Rays
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Cathode Rays
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H
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H+
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e–
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O →
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O+
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e–
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O2 →
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O2+
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e–
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O2 →
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O22+
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2e–
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The mass of a proton is very slightly less than that of a H-atom. This shows that protons are sub-atomic particles. Protons are fundamental constituent of matter because positive rays are produced by all substances.
“A sub-atomic particle, which is a fundamental constituent of all matter having a mass slightly less than that of H-atom and which carries a positive charge equal in magnitude to the charge on an electron, is called a proton”. A proton is denoted by p or p+ of +1p.
Comparison of Positive (Canal) Rays and Cathode Rays:
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Properties
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Cathode Rays
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Canal Rays
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Sign of Charge
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Negative
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Positive
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Mag. of Charge
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Always –1
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Mostly +1, but also +2, +3…
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Mass
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Definite value
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Variable, depends on ions
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e/m
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Definite value
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Variable, depends on ions
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