Composition of atom

Category : JEE Main & Advanced

             The works of J.J. Thomson and Ernst Rutherford actually laid the foundation of the modern picture of the atom. It is now believed that the atom consists of several sub-atomic particles like electron, proton, neutron, positron, neutrino, meson etc. Out of these particles, the electron, proton and the neutron are called fundamental subatomic particles and others are non-fundamental particles. Electron (?1eo)           

(1) It was discovered by J.J. Thomson (1897) and is negatively charged particle. Electron is a component particle of cathode rays.           

(2) Cathode rays were discovered by William Crooke's & J.J. Thomson (1880) using a cylindrical hard glass tube fitted with two metallic electrodes. The tube has a side tube with a stop cock. This tube was known as discharge tube. They passed electricity (10,000V) through a discharge tube at very low pressure (\[{{10}^{-2}}\] to \[{{10}^{-3}}mm\,Hg)\]. Blue rays were emerged from the cathode. These rays were termed as Cathode rays.           

(3) Properties of Cathode rays         

(i) Cathode rays travel in straight line.           

(ii) Cathode rays produce mechanical effect, as they can rotate the wheel placed in their path.           

(iii) Cathode rays consist of negatively charged particles known as electron.           

(iv) Cathode rays travel with high speed approaching that of light (ranging between \[{{10}^{-9}}\] to \[{{10}^{-11}}\] cm/sec)           

(v) Cathode rays can cause fluorescence.           

(vi) Cathode rays heat the object on which they fall due to transfer of kinetic energy to the object.           

(vii)  When cathode rays fall on solids such as \[Cu,\,X-\]rays are produced.           

(viii) Cathode rays possess ionizing power i.e., they ionize the gas through which they pass.           

(ix) The cathode rays produce scintillation on the photographic plates.           

(x) They can penetrate through thin metallic sheets.           

(xi) The nature of these rays does not depend upon the nature of gas or the cathode material used in discharge tube.           

(xii) The e/m (charge to mass ratio) for cathode rays was found to be the same as that for an \[{{e}^{-}}\] \[(-1.76\times {{10}^{8}}\] coloumb per gm). Thus, the cathode rays are a stream of electrons.

(xiii) According to Einstein?s theory of relativity, mass of electron in motion is, \[{m}'\] \[=\frac{\text{Rest mass of electron(m) }}{\sqrt{[1-{{(u/c)}^{2}}]}}\] Where \[u\]= velocity of electron, c= velocity of light.           

When    u=c  than mass of moving electron =¥.   Proton (1H1, H+, P)

(1) Proton was discovered by Goldstein and is positively charged particle. It is a component particle of anode rays.

(2) Goldstein (1886) used perforated cathode in the discharge tube and repeated Thomson's experiment and observed the formation of anode rays. These rays also termed as positive or canal rays.

(3) Properties of anode rays           

(i) Anode rays travel in straight line.

(ii) Anode rays are material particles.

(iii) Anode rays are positively charged. Comparison of mass, charge and specific charge of electron, proton and neutron  

Name of constant Unit Electron(e?) Proton(p+) Neutron(n)
  Mass (m) Amu Kg Relative 0.000546 9.109 × 10?31 1/1837 1.00728 1.673 × 10?27 1 1.00899 1.675 × 10?27 1
  Charge(e)   Coulomb (C) Esu Relative ? 1.602 × 10?19 ? 4.8 × 10?10 ? 1 +1.602 × 10?19 +4.8 × 10?10 +1 Zero Zero Zero
Specific charge (e/m) C/g 1.76 × 108 9.58 × 104 Zero
Density Gram / cc \[2.17\times {{10}^{-17}}\] \[1.114\times {{10}^{14}}\] \[1.5\times {{10}^{-14}}\].


The atomic mass unit (amu) is 1/12 of the mass of an individual atom of \[_{6}{{C}^{12}}\], i.e. \[1.660\times {{10}^{-27}}kg\].   Other non fundamental particles

Particle Symbol Nature Charge esu ´10?10 Mass (amu) Discovered by
Positron + + 4.8029 0.0005486 Anderson (1932)
Neutrino n 0 0 < 0.00002 Pauli (1933) and Fermi (1934)
Anti-proton ? ? 4.8029 1.00787 Chamberlain Sugri (1956)  and Weighland (1955)
Positive mu meson + + 4.8029 0.1152 Yukawa (1935)
Negative mu meson ? ? 4.8029 0.1152 Anderson (1937)
Positive pi meson + + 4.8029 0.1514  Powell (1947)      
Negative pi meson ? ? 4.8029 0.1514
Neutral pi meson 0 0 0.1454



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