How Did Rutherford Discover The Presence Of Nucleus In The Atom With The Help Of Alpha Particles. What Were The Main Defects In Rutherford's Atomic Model?

. In order to determine the structure of atom Rutherford carried out an experiment in 1911. He bombarded a very think gold foil ( 0.000004 cm thick) with alpha particles from a radio active source.

A photographic plate( or fluorescent screen) was placed behind the gold foil to observe the extent of scattering of alpha particles. The flashes on the screen caused by the individual (alpha) particles were counted to determine the relative number of alpha particles deflected at various angles. From the flashes on the photographic plate , it was found that nearly all the alpha particles passed through the gold foil with little or no deflection. However, a few alpha particles were deflected through large angles. A few alpha particles even bounced back from the old foil.

From this experiment Rutherford concluded that scattering of alpha particles were caused by the repulsion from a positive charged centre in the gold foil. This central heavy part of the atom was called nucleus.

Drawbacks
1) According to the classical electromagnetic theory , the revolving electrons will lose energy continuously . therefore, the orbit of the elements will become smaller and smaller until the electron fall into the nucleus.
2) If the electron emit energy continuously , it should form a continuous spectrum but actually line spectrum is obtained.

Rutherford's experiment was quite simple. He made alpha particles which are positively charged to pass through a thin gold foil. The result was that he found very few alpha particles reflected back while a few deviated through a large angle and major part of them passed without any deviation.

With this observation, Rutherford came to a conclusion that very few alpha particles reflected back due to the presence of a dense core which is positively charged(because of which the alpha particles were repeled).

The alpha particles that were deviated by a large angle hit the tip of the core(nucleus). Major part of the particles passed through without any deviation since there was more empty space around the nucleus.

The main defects in his atomic model was that the presence of neutrons were not identified. Secondly, he said electrons were around the nucleus but could not mention exactly how and why were they around the nucleus.

But Bohr's atomic model overcame these defects.