Model of atom | Thomson's | Rutherford's | Bohr's | Summerteld | Quantum mechanical

Thomson's Model of atom:

J.J.Thomson's was the first to propose a detailed model of atom in 1897. He proposed that that an atom consists of uniform positively charged sphare having the radius (1A°=10^-10 m) in which the electrically neutral . The total mass of an atom is spread over the atom. This model is also known as plum-pudding model.

Limitation of Thomson's Model of atom

(1) Identity of positively charged particles was not explained.

(2) This model was found wrong according to Rutherford's experiment on scatting of Alpha particles.Rutherford's Experiment on scattering of Alpha

Rutherford's experiment on scattering of Alpha

Rutherford and his co-workers carried out a series of experiment using Alpha particals. A beam of Alpha particles was bombarded against a thin foil of about 100 nm of gold, platinum, silver, copper, respectively.

The gold foil watch surrounded circular fluorescent ZnS screen whenever and Alpha particle struck the gold foil, it produces a flash of light.

Observations:- Rutherford observed the following points:-

(1) Most of the Alpha particles (about 99%) passed through the gold foil undeflected in a straight line.

(2) A few Alpha particles (about 1 in 10,000) deflected with a certain angle.

(3) A very few Alpha particles ( about 1 in 20,000) rebounce back its initial path.

(4) The deflection of Alpha particles was greather near the central portion of the atom.

Conclusion:- Rutherford found the following result about the atom:-

(1) Most of portion of an atom is empty as most of the Alpha particles passed through the gold foil undeflected in a straight line.

(2) Total mass and total positive charge of an atom is present in its Central portion called nucleus.

(3) The size of nucleus is negligible in comparision to the size of atom.

Rutherford's Atomic Model

On the basis of his Alpha particle scattering experiment, Rutherford proposed the atomic model called rutherford's Atomic model or Nuclear Atomic model. The main points of this model have been given below:-

(1) Most of the portion of an atom is empty.

(2) Total positive charge and total mass of an atom are present in its Central portion called "Nucleus".

(3) An atom consist of a heavy positively charged nucleus in which all the protons(p+) and neutrons (n) are present. Protons(p+) and neutrons (n) are collectively known as "nucleons".  i.e Nucleons= proton+ neutron = Atomic mass(A)

(4) The size of nucleus is about 10^-5th of  the size atom.

Radius of nucleus/Radius of atom=10^-13cm/10^-8cm= 10^-5

(5) the volume of nucleus is about 10^-5th of the volume of atom.

Volume of nucleus/volume of atom=(10^-13cm)³/(10^-8cm)³=10^-15

(6) The density of nucleus of an atom may be calculated as:-

(7) There is an empty space around the nucleus of an atom, called extranuclear part. In this part, electrons revolve in a circular path called "orbit".

(8) An atom is electrically neutral because total number of protons (p+) present in the nucleus is always equal to the total number of electrons (e-) around the nucleus.

Atomic no.(z)=p+=e-

(9) Electrons revolve around the nucleus in a closed orbit with very high speed. The centrifugal force acting on revolving electron is being Counter balance by the force of attraction between the electron (e-) and nucleus.

(10) Rutherford, compare the atom with solar system in which nucleus representing the sum and revolving electrons as planets. These electrons are called planetry electrons.

Drawback's of Rutherford's Atomic Model

(1) Rutherford's Atomic model could not explain the stability of atom.

[According to Neils Bohr, when the electron (e-) revolves around the nucleus, it should loss energy in the the form of electro magnetic radiation and it should come closer and closer to the nucleus following the spiral path and ultimately the electron should fall into the nucleus therehy making the atom unstable i.e the atom would collapse. But, actually it does not so as atom is stable.

(2) This theory could not explain the distribution (or arrangement) of electron into the various orbit of an atom.

(3) This theory could not explain the hydrogen spectrum.

(4) Dissimilarities between solar system and nuclear atomic model:

=>The sun and planets are very big bodies and uncharged while the nucleus and electrons are microscopic object and charged.

=>The revolution of planets around the sun is governed by gravitational forces of attraction while the revolution of electron around the nucleus is governed by electrostatic forces of attraction.

=>There is only one planet which can revolve in a particular orbit around the sun but in an atom, more than one electron can revolve in a particular orbit of the atom.