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Niels Bohr gave some postulates describing the structure of an atom

1. In an atom, the electrons revolve around the nucleus in certain definite circular paths called orbits, or shells. 2. Each shell or orbit corresponds to a definite energy. Therefore, these circular orbits are also known as energy levels or energy shells. 3. The orbits or energy levels are characterized by an integer not, where, n can have values 1, 2, 3, 4……. The integer not (= 1, 2, 3…) is called the quantum number of respective orbit. The orbits are numbered as 1, 2, 3, 4………… etc., starting from the nucleus side. Thus, the orbit for which n=1 is the lowest energy level. The orbits corresponding to n = 1,2,3,4…..etc., are also designated as K,L,M,N……….etc., shells. When the electron is in the lowest energy level, it is said to be in the ground state. Since, electronics can be present only in these orbits, hence, these electrons can only have energies corresponding to these energy levels, i.e., electrons in an atom can have only certain permissible energies . 4. The electrons present in an atom can move from a lower energy level (Elower) to a level of higher energy (Ehigher) by absorbing the appropriate energy. Similarly, an electron can jump from a higher energy level (Ehigher) to a lower energy level (Elower) by losing the appropriate energy. The energy absorbed or lost is equal to the difference between the energies of the two energy levels, i.e., ΔE= Ehigher - Elower

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Neils Bohr postulates:Bohr recognised that lines in atomic spectrum are related to the arrangement of electrons in that atom.
1)electrons revolve around the nucleus with definite velocities in specified paths called orbits.the energy of the electron remains same when it is present in the orbit.it neither emits nor absorbs energy.these orbits are called stationary orbits.
2)Bohr proposed that angular momentum of an electron is quantised.i.e.,mvr=nh/2pi[h is planck's constant,m is mass of electron,v is velocity of electron,r is radius of orbit,n is any simple integer].the stationary states are only those where n=1,2,3 or 4...this is called Bohr's quantum condition.
3)the energy of an electron changes only when an electron moves from one orbit to another one.an electron transition from an inner orbit to outer one is possible on absorption of energy whereas from outer to inner results in release of energy..i.e.,E2-E1=deltaE=h(nu)[where E2 is energy of outer orbit & E1 is energy of inner orbit].'n' is termed as principal quantum no. by which Bohr calculated radii of various orbits and energies of electrons in those orbits.he named orbits also as K,L,M...energy levels respectively.
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