CHEMISTRY - Models of Atom

<b> Thomson Model of Atom :-</b> J.J. Thomson, in 1904, proposed that an atom was a sphere of positive electricityin which were embedded number of electrons, sufficient to neutralize the positive charge. The stability of the atom was explained as a result of the balance between the repulsive forces between the electrons and their attraction towards the centre of the positive sphere. This model is compared with a water melon in which seeds are embended or with a cake or pudding in which raisins are embedded . That is why this model is sometimes called raisin pudding model or watermelon model.

<b>Limitation :-</b> Though this model was able to explain the overall neutrality of the atom, it could not satisfactorily explain the results of scattering experiments, carried out by Rutherford in 1911 and was rejected.

<b> Rutherford's Model ( Nuclear Model ) of Atom - Discovery of Nucleus :-</b> Rutherford, in 1911, performed some scattering experiments in which he bombarded thin foils of metals like gold, silver, platinum or copper with a beam of fast moving alpha - particles. The source of alpha - particles was radium, a radioactive substance, placed in a block of lead. Slits were used to get a fine beam. The presence of alpha-particle at any point around the thin foil of gold after striking it was detected with the help of a circular zinc sulphide screen. The point at which an alpha-particle strikes this screen, a flash of light is given out. From these experiments, he made the following observations :-

* Most of the alpha- particles passed through the foil without undergoing any deflection.

* Few alpha-particles underwent deflection through small angles.

* Very few were defected back, i.e through an angle greater than 90 degree.

From these observations, Rutherford drew the following conclusions :-

* Since most of the alpha-particles passed through the foil without undergoing any deflection, there must be sufficient empty space within the atom.

* Since few alpha-particles were deflected through small angle and alpha-particles were positively charged particles, these could be deflected only by some positive body present within the atom.

* Since some alpha-particles were deflected back and alpha-particles are heavy particles, these could be deflected back only when they strike some heavier  body inside the atom.

* Since the number of alpha-particles deflected back is very very small, this shows that the heavy body present in the atom must be occupying a very very small volume.

<b> Bohr's Model of Atom :-</b> To overcome the drawback of Rutherford's  model of atom and to explain the line spectrum of hydrogen, Neils Bohar, a Danish physicist in 1913 proposed a new model of atom based upon Planck's quantum theory. This new model is called Bohr's model of atom.

Postulates of Bohr's Model of Atom :-
* An atom consist of a small, heavy positively charged nucleus in the centre and the electron revolve around it in circular orbits.

* Out of large number of circular orbits theoretically possible around the nucleus, the electron revolve only in those orbits which have a fixed value of energy. Hence, these orbits are called energy level or stationary states. The word stationary does not mean that the electrons are stationary but it means that the energy of the electron revolving in a particular orbit is fixed and does not change with time.