Thomson Model –
JJ Thomson proposed that atom is a square of the positively charged metal in which electrons are embedded. He compare his model with watermelon in which seeds are the electrons and entire watermelon is an atom . The mass of atom is uniformly distributed. This model is known as plum pudding model.
Rutherford Model For Metal –
Alpha particle scattering experiment – An Alpha particle is a heme iron which has two electrons removed. In this experiment a ray of Alpha particle is provided with small opening or narrow opening. This beam is allowed to fall on a thin gold foil of thickness 2.1 x 10 raise to power -7 meter. The Alpha particles are scattered in different directions which are observed by a rotatable detector in which a plate of zinc sulphite and microscope is placed.
Result – On the basis of scattering of Alpha particles following results are found –
- Most of the Alpha particles pass straight to the gold foil.
- Some of them are deflected by a small angle.
- A few Alpha particles ( one in eight thousand) are deflected through the angle more than the 90 degrees.
- A few particles returned back and deflected by the angle of 180 degrees.
Conclusion – By Alpha particles experiment Rutherford concluded the following points regarding atom –
- Most of the mass and all the charge of an atom concentrated in a very small region which is known as the nucleus.
- The nucleus is the positively charged and its size is of the order of 10 raised to the power minus 15 meters.
- In an atom, there is maximum empty space and the electron revolves around the nucleus in the same way as the planets revolve around the sun.
Drawbacks of Rutherford’s Model –
According to the concepts of the electrodynamic if a charged particles revolves within acceleration. It radiates energy in the form of electro magnetic wave an electron moving a circular path around the nucleus should also radiate energy and thus move into smaller and smaller orbit gradually e decreasing radius and it shall ultimately for into nucleus. The energy spectrum of an electron should be continuous but practically The spectrum is linear. It did not explain the dissociation of an electron outside the nucleus.
The closest distance of approach – The distance from the nucleus at which total kinetic energy of Alpha particles are converted into electricity tension energy that is minimum distance is known as the closest distance of approach.
In Rutherford model could not elaborate on the stability of an atom. So Bohr because of the different postulates according to the atomic model.
Bohr’s Postulates –
- According to the model, the electrons revolve around the nucleus is stationary orbits in which necessary centripetal force is provided by the Coulomb force.
- The electron can revolve only in certain discrete non-radiating orbits called stationary orbits for which the total angular momentum of the revolving electron is an integral multiple.
- The radiation of energy e occurs only when an electron jumps from one permitted to another. when an electron jumps from higher energy orbital to the lower energy which then the difference of the energy levels is admitted in the form of the photon but if electron goes from lower energy orbit to the higher energy orbit it absorbs the same amount of energy.
Note – The Bohr Modal was proposed for hydrogen-like atoms for which z is equal to one.
The Radius Of An Orbit –
Consider in a hydrogen atom electron is revolving around the nucleus in a stationary orbit of radius r then for an orbit then according to the bohr postulates.
Note – For first Bohr orbit of hydrogen atom the ratio of the speed of an electron to the speed of the light is called the fine structure constant.
Explanation Of Hydrogen Spectrum –
If the hydrogen gas is closed in a vacuum tube and if it is heated or current is passed to the gas then the radiations are produced due to the ionization of the gas. If The spectrum is seen by the spectrometer these are the certain wavelengths which are present in the spectrum of hydrogen. Different band of wavelength are explain which are lyman series, Balmer series, paschen series, bracket series and P fund series. Lyman and balmer series are seen discontinuous while the spectrum of paschen series is found continuous. So The spectrum of hydrogen is linear and discontinuous.