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Question

Which of these are Bohr's postulates?

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Solution

The correct options are

**A** In an atom, the electrons revolve around the nucleus in certain definite circular paths called stationary orbits in which they do not radiate energy.

**B** Electron revolves around nucleus only in those orbits for which the angular momentum is an integral multiple of

**D** An electron present in an atom can move from a lower energy level to a level of higher energy by absorbing the appropriate energy.

The following are Bohr's postulates

The following are Bohr's postulates

(i) Bohr’s first postulate was that an electron in an atom could revolve in certain stable orbits without the emission of energy, contrary to the predictions of electromagnetic theory. According to this postulate, each atom has certain definite stable states in which it can exist, and each possible state has definite total energy. These are called the stationary states of the atom.

(ii) Bohr’s second postulate incorporated into atomic theory the early quantum concepts that had been developed by Planck and Einstein. It states that an electron might make a transition from one of its specified non-radiating orbits to another of lower energy. When it does so, a photon is emitted having energy equal to the energy difference between the initial and final states. The frequency of the emitted photon is then given by

hv = Ei - Ef

where Ei and Ef are the energies of the initial and final states and Ei > Ef

(iii) Bohr’s third postulate defines these stable orbits. This postulate states that the electron revolves around the nucleus only in those orbits for which the angular momentum is some integral multiple of h2π where h is the Planck’s constant (=6.6×10−34)J Thus the angular momentum (L) of the orbiting electron is quantised. That is L=nh2π

His postulates were true for hydrogen or hydrogen like atoms with single electron like (He+,Li++)

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