Subatomic Particles - Electron

Chemistry

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Discovery of electron by J.J. Thomson

 In 1878 William Crooks carried out discharge tube experiments and discovered new radiations and called them cathode rays. Since these rays travel from the cathode towards anode. Later, J.J Thomson studied the characteristics of cathode rays and concluded that cathode rays are negatively charged particles, now called electrons. The name electron was given by Johnson Stoney.

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Properties of Cathode rays

1. Cathode rays travel in straight lines. That is why, cathode rays cast shadow of any solid object placed in their path. The path cathode rays travel is not affected by the position of the anode.

2. Cathode rays consist of matter particles, and posses energy by the virtue of its mass and velocity. Cathode rays set a paddle wheel into motion when it is placed in the path of these rays one the bladder of the paddle wheel.

3. Cathode rays consist of negatively charged particles. When cathode rays are subjected to an electrical field, these get deflected towards the positively charge plate(Anode).

4. Cathode rays heat the object only which they fall. The cathode ray particles possess kinetic energy. When these particles strike an object, a part of the kinetic energy is transferred to the object. The causes a rise in the temperature of the object.

5. Cathode rays cause green fluorescence on glass surface, i.e., the glass surface only which the cathode rays strike show a colored shine.

6. Cathode rays can penetrate through thin metallic sheets.

7. Cathode rays ionize the gases through which they travel.

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Electrical nature of matter

Matter is electrically neutral. It contains the equal number of protons and electrons that make the matter neutral. If there is a dis-balance in any one of them then the matter is positively or negatively charged depending on whether proton or electrons are more in number.

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Discovery of electrons by William Crookes

William Crookes in 1879, studied the conduction of electricity through a gases at low pressure. For this purpose, he took a discharge tube which is a long glass tube, about 60 cm long, sealed at both the ends and fitted with metal electrodes. It has a side tube fitted with a stop cock which can be connected to a vacuum pump to reduce the pressure of the gas inside to any desired value. These tubes are also now called as Crookes tube.

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Electron and its properties

Electron is the negative part of the matter. It is negatively charged. The properties of electrons are:
  • They are produced by the negative electrode, or cathode, in an evacuated tube, and travel towards the anode.
  • They travel in straight lines and cast sharp shadows.
  • They have energy and can do work.
  • They are deflected by electric and magnetic fields and have a negative charge.

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Millikan Oil's drop experiment

  • Millikan's oil drop experiment measured the charge of an electron. Before this experiment, existence of subatomic particles, was not universally accepted. Millikan's apparatus contained an electric field created between a parallel pair of metal plates, which were held apart by insulating material. Electrically charged oil droplets entered the electric field and were balanced between two plates by altering the field.
  • When the charged drops fell at a constant rate, the gravitational and electric forces on it were equal. Therefore, the charge on the oil drop was calculated using formula. Millikan found that the charge of a single electron was C.
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    Postulates of discovery of electrons

    Postulates of Thomsons Model: an atom consists of a positively charged sphere, with electrons set within the sphere. An atom is electrically neutral as the positive and negative charges within it are equal.

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    Charged particles

    The charged particles are the particles  which carries positive or negative charge on them. Ions could be positively or negatively charged, based on either they lose or gain the electrons in the outermost shell of their atom.

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    Properties of Electrons

    Once people got to know that subatomic particles exist, many scientists began to study electrical discharge in partially evacuated tubes, known as 'Cathode Ray Discharge Tubes'. Cathode rays move from negative electrode (cathode) to positive electrode (anode). Results of these experiments shed some light on physical properties of electrons which are summarized below:
    1. Electrons are negatively charged particles and they travel from cathode to anode.
    2. They are very light as compared to atom.
    3. Nature of electrons do not depend on the material. They posses the same set of characteristics universally. Hence, electrons are basic constituent of all atoms.

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