Question

What are the postulates of Bohr’s model of an atom?

Answer 1

Hint: Bohr proposed his model of an atom and its postulates to overcome the drawbacks of Rutherford’s model and to explain the line spectrum of hydrogen. Bohr's model of an atom mainly tells about the energy levels in the atom.


Complete step by step answer:
The main postulates of Bohr’s model of an atom are given below:
(i) In an atom, there are small, heavy positively charged nuclei in the center and the electrons revolve around it in circular orbits.
(ii) Out of a large number of circular orbits theoretically possible around the nucleus, the electrons revolve only in those orbits which have a fixed value of energy. Hence, we call these orbits as energy levels or stationary states. The word stationery does not mean that the electrons are stationary but it means that the energy revolving in a particular orbit is fixed and does not change with time. The different energy levels are numbered as 1, 2, 3, 4….etc or we can designate them as K, L, M, N, O, P…..etc, starting from the closest to the nucleus.
(iii) Since the electrons revolve only in those orbits which have fixed values of energy, hence electrons in an atom can have only certain definite or discrete values of energy and not any value of their own. So, we can say that the energy of an electron is quantized.
(iv) The angular momentum of an electron in an atom must have certain definite or discrete values and it should not have any value of its own. We can express angular momentum by only permissible values given in the expression:
$mvr=\dfrac{nh}{2\pi }$
(v) When the electrons in an atom are in their lowest energy state, they keep on revolving in their respective orbits without losing energy can neither be lost nor gained continuously. This state of an atom is called a normal or ground state.
(vi) When the electron jumps from one orbit to the other only then energy is emitted or absorbed.

Note: The energy of the different energy states in the case of the hydrogen atom are given by the expression (called Bohr formula):
${{E}_{n}}=-\dfrac{2{{\pi }^{2}}m{{e}^{4}}}{{{n}^{2}}{{h}^{2}}}$
When we substitute the values of m (mass of an electron), e (charge on the electron), and h (Planck's constant) the expression becomes:
${{E}_{n}}=-\dfrac{21.8\text{ x 1}{{\text{0}}^{-19}}}{{{n}^{2}}}$ .