Rutherford model of an atom and bohr's atomic model postulates

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Rutherford model of an atom and boards atomic model postulated


Rutherford model of an atom

On the basis of the alpha particle scattering experiment, Rutherford proposed the following model of an atom:-

(1). An atom consist of a small and massive Central core in which the entire positive charge of the atom are concentrated. This core is called the nucleus. 

(2). The size of the nucleus is about 10( of power of -15 ) m Or 1 Fermimeter and the size of atom is the order of 10(of power of -10) m. 

(3). The nucleus is surrounded by a suitable no. Of electrons so that there negative charge is equal to the positive charge of nucleus and the atom as a whole is electrically neutral. 

(4). The electrons revolve around the nucleus in various orbits. The centripetal force required for their revolution is provided by electrostatic force of attraction between electrons and nucleus. 

Limitations of Rutherford atomic model

(1). According to electromagnetic theory an accelerated charge Particle must radiate electromagnetic energy as electrons revolves around the nucleus then it continuously losses it's energy and finally it should collapse into the nucleus due to it's spiral motion around the nucleus though, the Rutherford model cannot explain the stability of an atom. 

(2). According to Rutherford the electrons should emits continous spectrum but an atom like hydrogen always emits a discrete line spectrum. 
Thus, it cannot explain the spectrum of various atoms. 

Bohr's atomic model postulates

(1).  An atom consist a small positively charged massive core known as nucleus electrons revolves around the nucleus. The attraction force between electrons and nucleus provided necessary centripetal force. 

(2). According to bohr,  electrons are revolve only in those orbits in which the angular momentum of electrons, is an integral multiply of h/2π.

W. K. T

Angular momentum = L = mvr

L = mvr = nh/2π
Where, n = 1,2,3, _ _ _ _ 

(3). Electrons revolves around the nucleus in a fixed orbit  having definite energies during revolution electrons does not radiates energy these orbits are called stationary orbits. 

(4). An electron can emit or absorb energy in the form of photons only when an electron jumps from higher to the lower orbit or from a Lower to higher orbit. 
If E1 and E2  be the energies associated with two orbits (E2 is greater than E1) then according to bohr's
               E2 - E1 = hv
    Where, hv = energy of photon

It is known as bohr's famous frequency condition. 

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