RUTHERFORD ‘ S MODEL OF AN ATOM

Ernst Rutherford Rutherford was born at Nelson , New

Zealand. He was educated at University of New Zealand and conducted research work in 1895 with J . J . Thomson at the Cavendish Laboratory . In 1907 , he accepted Longworthy Professorship at Manchester University in England where Chawdick was one of his students. He won Noble Prize in 1908 in the field of atomic physics.

He explained that the major portion of the atomic mass is concentrated in a minute , centrally located , positively charged nucleus. As early as in 1920 , he predicted the presence of particles with unit mass but having no charge . He named such particles neutrons.

HISTORY Rutherford scattering is a

phenomenon in physics that was explained by Ernest Rutherford in 1911, which led to the development of the Rutherford model (planetary model) of the atom, and eventually to the Bohr model. It is now exploited by the materials analytical technique Rutherford backscattering.

Conclusions drawn by Rutherford

Rutherford concluded from the alpha particles scattering experiment that –

a) Most of the space inside the atom is empty because most of the alpha particles passed through the gold foil without getting deflected.

b) Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.

c) A very small fraction of alpha particles were deflected by 180o , indicating that all the positive charges and mass of the gold atom were concentrated in a very small volume within the atom

Rutherford’s Nuclear Model of an Atom The main points of Rutherford’s

nuclear model are described below :a) An atom consists of a positively

charged nucleus, which is surrounded by electrons moving around it.

b) Electrons and the nucleus are held together by coulombic force of attraction .

c) The size of nucleus is very small as compared to the size of atom . Experimentally, it was found that ,

Radius of the nucleus of an atom=10-14 to 10-

15m Radius of an atom =10-10mThus, the size of the nucleus is about ten-

thousandth part of the size of an atom.d) Almost the entire mass of an atom is

concentrated in its nucleus.e) Atom , as a whole , is electrically neutral.

So , number of protons inside the nucleus of an atom and the number of electrons surrounding the nucleus are equal .

How Rutherford’s atomic model Is different from Thomson atomic model ????

Rutherford proposed a model in which electrons revolve around the nucleus in well – defined orbits . There is a positively charged centre in an atom called the Nucleus . He also proposed that the size of the nucleus is very small as compared to the size of the atom and nearly all the mass of an atom is centred in the nucleus.

Thomson proposed the model of an atom to be similar to a christmas pudding. The electros are in a positively charged sphere like christmas pudding and the mass of the atom was supposed to be uniformly disturbed .

Drawbacks of Rutherford The Rutherford’s atomic model

explains the structure of an atom in a very simple way. But ,it suffers from the following drawbacks :

a) An electron revolving around the nucleus gets accelerated towards the nucleus. An accelerating charged particle must emit radiation , and lose energy .Thus , the electrons in an atom must continuously emit radiation and lose energy .

Because of this lose energy , the electron would slow down , and will not be able to withstand and the attraction of the nucleus . As a result , the electron should follow a spiral path , and ultimately fall into the nucleus. If it happens , then the atom should collapse in about 10-8 second . But, this does not happen – atoms are stable . This indicates that there is something wrong in Rutherford’s nuclear model of atom.

b) Rutherford’s model of atom does not say anything about the arrangement of electron in an atom.