POSTULATES - In a nucleus of an atom proton and neutrons are present....

2) electron revolve fixed energy level ....

3) BT electron revolve only that energy level in which its angular momentum is multiple of h/2π

=>In 1913, Bohr put forward a theory based on quantization of energy to improve upon the Rutherfords model of the structure of atom. This theory also satisfactorily explained the line spectrum of hydrogen atom. He postulated that: 1)The electrons move around the nucleus in one of the several fixed circular orbits calledstationary energy levels. These energy levels are arranged concentrically around the nucleus, and are characterized by an integer n, the lowest level being given the number 1. The energy level corresponding to n=1, 2, 3, 4, are also known as K, L, M, N shells. 2)Electrons can move about only in certain orbits which have specific energies. Their movement is possible in only those orbits for which itsangular momentum is an integral multiple of h/2Πor mvr=nh/2Π where n is any integer 1, 2, 3, 4.n. 3)The energy level nearer to the nucleus has low energy, where as that farthest from it has maximum energy. An electron is said to be in ground state, when it moves in energy level having lowest energy. The ground state is the most stable state of the atom. The electron has a definite energy which is characteristic of the orbit in which it is moving. As long as the electron remains in an orbit, it does not lose energy. These orbits are hence calledstationary orbits. 4)Energy is emitted or absorbed when an electron moves from one level to another. Thus by absorbing one particular quantum of energy, the electron will jump from a energy level 1 to 2 or 2 to 3. It is then said to be in excited state. The quantum of energy absorbed in each case is equal to the difference in energies of the two levels. An electron cannot have an energy that would place it in between the two permissible orbits. 5)When an electron moves from a higher energy (E2) orbit to a lower energy (E1) orbit, the energy (ΔE = E2 E1) is emitted in the form of a photon of frequency v such that ΔE=E2-E1=hv