THOMSON MODEL OF ATOM * J.J THOMSON in 1898,he proposed that an atom possesses a spherical shape (radius approximately 10 raise -10m) in which the positive charge is uniformly distributed.

* The electrons are embedded into it in such a manner as to give the most stable electrostatic arrangement. Many different names are given to this model,for example, plum pudding, raisin pudding or watermelon.This model can be visualized as a pudding or watermelon of positive charge with plums or seeds (electrons) embedded into it. An important feature of this model is that the mass of the atom is assumed to be uniformly distributed over the atom. Although this model was able to explain the overall neutrality of the atom,but was not consistent with the results of later experiment. Sir J.J Thomson

According to this model:- 1)The positive charge and most of the mass of the atom was densely concentrated in extremely small region. This very small portion of the atom was called nucleus. 2) The nucleus is surrounded by electrons that move around the nucleus with a very high speed in circular paths called orbits. 3) Electrons and the nucleus are held together by electrostatic forces of attraction. RUTHERFORD’S NUCLEAR MODEL OF ATOM

RUTHERFORD’S SCATTERING EXPERIMENT

SCHEMATIC MOLECULAR VIEW OF THE GOLD FOIL

OBSERVATIONS:- 1)Most of the alpha particles passed through the gold foil undeflected. 2) A small fraction of the alpha particles was deflected by small angles. 3) A very few alpha particles (approx. 1 in 20,000) bounded back,that is, were deflected nearly 180 degree.

1) Most of the space in the atom is empty as most of the alpha particles passed through the foil undeflected. 2) A few positively charged alpha particles were deflected. The deflection must be due to enormous repulsive force showing that the positive of the atom is not spread throughout the atom as Thomson had presumed the positive charge has to be concentrated in a very small volume that repelled and deflected the positively charged alpha particle. 3) Calculation by Rutherford showed the volume occupied by the nucleus is negligibly small as compared to the total volume of the atom. The radius of the atom is about 10 raise -10m, while that of nucleus is 10 raise - 15m.one can appreciate this difference in size by realizing that if a cricket ball represents in a nucleus than the radius of the atom would be about 5km.

1) The positive charge and most of the mass of the atom was densely concentrated in extremely small region. This very small portion of the atom was called nucleus by Rutherford. 2) The nucleus is surrounded by electrons that move around the nucleus with a very high speed in circular paths called orbits. Thus Rutherford’s model of atom resembles the solar system in which the nucleus plays a role of sun and the electrons that of revolving planets. 3) Electrons and the nucleus are held together by electrostatic forces of attraction.

1) It could not explain stability of an atom. 2) According to eletromagnetic theory it charge body is present in a motion in it should be emits some radiation finally it should fall in the nucleus. 3) He could not explain about electronic structure of an atom.

* DEVELOPMENT LEADING TO THE BOHR’S MODEL OF ATOM 1) Dual character of the electromagnetic radiation which means that radiation possess both wave like and particle like properties. 2) Experimental results regarding atomic spectra which can be explained only by assuming quantized eletronics levels in atoms.

1) Electrons are revolve around the nucleus in fixed energy circular path known as orbits. 2) The energy of electrons does not changed until it remains same orbit if it absorb energy a jump of higher orbit and it returned to original state then it emit energy. 3) A frequency of radiation absorbed or emitted is equal to, V = ∆E / h. 4) The angular momentmum of an electrons is an integral multiple of :- mvr = nh/ 2 ∏.

LIMITATIONS OF BOHR’S MODEL 1) It could be explained only he could explained single electron system spectrum. 2) He could not explained splitting of lines in the presence of magnetic field known as zeeman, and electric field known as stark. 3) He could not explain fines spectra of hydrogen. if an electron completely remove then the energy known as ionization.

M ADE BY :- SAPNA – 22 MINAKSHI -14 GULSHAD-8 GITANJALI-7 CLASS- 11 SCIENCE