Discovery Of Electron
The existence of electrons in an atom was shown by J.J.Thomson in 1897.Thomson passed electricity at high voltage through a gas at very low pressure taken in a discharge tube.Streams of minute particles were given out by the cathode (negative electrode). These streams of particles are called cathode rays (because they come out of cathode). The mass and charge of the cathode ray particles does not depend on the nature of gas taken in the discharge tube.Cathode rays consist of small,negatively charged particles called electrons. Since all the gases form cathode rays, it was concluded that all the atoms contain negatively charged particles called electrons.
Thomson explained the formation of cathode rays as follows.The gas taken in the discharge tube consists of atoms,and all the atoms contain electrons.When high electrical voltage is applied,the electrical energy pushes out some of the electrons from the atoms of the gas.These fast moving electrons form cathode rays.Thus,the formation of cathode rays shows that one of the subatomic particle present in all the atoms is the negatively charged 'electron'.We can now define an electron as follows.
The electron is a negatively charged particle found in the atoms of all the elements.The electrons are located outside the nucleus in an atom. Only hydrogen atom contains one electron, all other atoms contain more than one electron. An electron is usually represented by the symbol e (e for electron and minus sign for negative charge).
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Structure of the Atom - Notes
1. Drawback of Rutherford’s Model of the Atom
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2. Rutherford’s Experiment - Discovery of Nucleus
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3. For the symbols H, D and T,tabulate three sub-atomic particles found in each of them.
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5. Structure of The Atom - Study Points
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10. Arrangement Of Electrons In The Atoms
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12. Limitations of Rutherford's model of the atom
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16. All about Names of the Chemical Elements
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17. How to learn naming Chemical Formulae?
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21. Comparison between Proton, Neutron and Electron
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22. Electronic Configurations of First 20 Elements
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26. Rules for writing of distribution of electrons in various shells for the first 18 elements
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36. The Physical Properties of the Isotopes of an Element are Different
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39. All the Isotopes of an Element Have Identical Chemical Properties
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42. Relationship Between Mass Number and Atomic Number
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44. Valaence Electrons (Or Valancy Electrons)
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45. Electronic Configurations of Noble Gases (or Inert Gases)
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48. Reason for the Fractional Atomic Masses of Elements
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49. Applications of Radioactive Isotopes
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