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MHT-CET : Physics Entrance Exam

MHT - CET : Physics - Atoms Molecules and Nuclei Page 1

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Emission of Spectral Lines


When chemical elements in the form of gas or vapour are excited by either heat or a strong electric discharge, they emit radiations of only certain wavelengths. When the radiation emitted by an element is observed by means of a spectroscope, bright-coloured lines (line spectrum) are seen. Each element emits its own spectrum.

It is observed that hydrogen is the lightest element and emits a line spectrum, which exhibits a definite regularity. The line spectrum of hydrogen consists of four lines in the visible region. These lines form a series because as the wavelength of line decreases, the separation between the lines decreases. Many attempts were made to explain these lines on the basis of classical theory.

According to Thomson's atom model, an atom is a sphere of positive charge in which electrons are embedded at definite sites. These sites are supposed to be distributed in such a way that the mutual electrostatic repulsion of the electrons is exactly balanced by their effective attraction towards the centre of the sphere which the positive charge exerts on them and the atom becomes stable.





This atom model failed because,

  1. Vibrating electrons give rise to continuous spectrum while it is found that any element gives a line spectrum.
  2. According to Rutherford's experiments, the positive charge of the atom is concentrated in a very small space. This is in contradiction to the idea of Thomson's atom model.




Rutherford Model of an Atom


Rutherford Atom Model

  • Rutherford model gives the idea about structure of the atom.
  • All positive charge and almost all mass is packed in a very small spherical volume
    (radius 10
    -14m) inside the atom. This sphere is called the nucleus (radius about 10-10m)
  • Electrons revolve around the nucleus.





Note : The radius of an atom is 10,000 times that of a nucleus.



Defects of Rutherford Model

1.      Electrons revolve around the nucleus in circular orbits. The circular motion is an accelerated motion. According to classical theory, accelerated charge radiates energy continuously. Therefore, orbiting electron which is an accelerated charge, radiates energy. Therefore, it loses energy and finally falls into the nucleus. Thus, the atom should be unstable, however, the atom is stable.

2.      As the electron loses energy, it moves along a spiral path and radius of the orbit decreases. As a result, the velocity and frequency of the electron go on increasing. Therefore, it should emit radiation continuously which should give rise to a continuous spectrum. However, one gets a line spectrum.



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