Thomson Model of Atom & It’s Drawbacks

Thomson Model of Atom :-


In 1888, Thomson suggested a model of atom. He proposed that the atom was spherical in shape and in that positive charges were uniformly distributed in this sphere of atomic dimension.
Sufficient number of electrons were supposed to be embedded in it to balance the positive charge. Thus the electron placed within the positive charge resembled a watermelon.
   

Fig. 1. Thomson model of atom 

 
He argued that within a single electron atom like Hydrogen atom, the electron must be situated at the center.

Fig 2. Hydrogen atom as per Thomson

In the diatom like Helium, the two electrons must be situated symmetrically on opposite sides of center at a distance equal to half of radius of positive sphere.

Fig 3. Helium atom as per Thomson

In the three electron atom, the atoms must be situated at the corners of  symmetrically placed equilateral triangle ,with sides equal to radius of sphere and so on.
   

Fig 4. Lithium atom as per Thomson. 

                                   
In order to account for emission of light, the electrons were allowed to oscillate or vibrate about their mean position. In this process, electrons radiates light whose frequency and wavelength is equal to frequency and wavelength given by experiment.

He suggested that when no light was being emitted, the electrons are at rest. Thus the Thomson’s atomic model explained successfully the existence of positively and negatively charged ion satisfied the requirement of stability of atom and explained the emission of electromagnetic waves.

Failures of Thomson Model:-

i). According to this model, Hydrogen can give rise to a single spectral line, while experimentally it was observed that Hydrogen spectrum contains several species (Lymann, Balmer, Paschen, Brackets, Pfund ) with several lines in each series. Thus Thomson model was unable to explain spectrum.

ii). Rutherford’s alpha particles scattering experiment proposed that whole mass of atom is concentrated in a very small region, i.e. at the centre of the atom. Thus, Thomson’s experiment of uniform distribution of positive charge was rejected later.

This is Thomson model of atom. In my next tutorial I’ll explain Scattering of alpha particles & Rutherford model of atom.Still any query write to us in comment section. 

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