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Rutherford's Model

In 1910, Rutherford carried out the experiment in which a thin sheet of gold foil was bombarded with a beam of α-particles. The results of the experiment were as follows:
  1. Most of the α -particles passed straight through the gold foil
  2. A few α-particles were deflected from the straight-line path
  3. A very small number of the α -particles (about 1 in 20,000) were deflected backwards through an angle greater than 90o
These observations are shown schematically in the figure below:
The atomic model as suggested by Thomson could not explain the above experimental observations. In order to account for these, Rutherford proposed that the atom was made up of a tiny nucleus, which carried a positive charge and practically the whole of the mass of the atom.

Around this nucleus there was a diffuse region containing electrons to maintain electrical neutrality to the atom. From the experiment, it was concluded that

Approximate radii of the nuclei of atoms
10-14 - 10-15 m
Approximate radii of outer hollow sphere containing electrons
10-10 m
On this basis, it has been calculated that less than 10-12 of the total volume of an atom is occupied by its nucleus.

Limitations of Rutherford's Model
In the Rutherford model of an atom, there exists a positively charged nucleus at the centre and it is surrounded by negatively charged electrons which move in definite paths, called orbits. Most of the mass of an atom is concentrated in the nucleus. 

Now according to the Maxwell theory of electromagnetic radiation, charged particles when accelerated should lose energy in the form of electromagnetic radiation. In the Rutherford model, negatively charged electrons move in orbits around the positively charged nucleus. In order to maintain the circular motion of an electron in orbit, the electron is subjected to acceleration. Thus, according to the Maxwell theory the electron should continuously lose energy and finally spiral into the nucleus with the subsequent collapse of the atom. Calculations show that the electron would require only 10-8 s to do so. Hence, such an arrangement of the nucleus and electrons will not yield a stable atom.

Besides the above difficulty, the Rutherford model could not explain the discrete spectra exhibited by atoms. By discrete spectra, we mean that an atom on excitation liberates characteristic radiations of fixed wavelengths. For example, high-voltage electrical discharge through sodium yields two yellow radiations of wavelengths 589.16 nm and 589.76 nm, respectively. Similarly, other gases exhibit characteristic wavelengths.

In 1885, Balmer analyzed the spectra of hydrogen and predicted that the observed lines in the visible and near ultraviolet spectrum could be expressed as

where n1 has a fixed value of 2, whereas n2 has integral values greater than 2, i.e. 3, 4, 5...

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