Question

Give any two postulates of Bohr’s model of atom.

Answer 1

Hint: Bohr proposed a model of the atom. According to the Bohr model of an atom, the negatively charged electrons are revolving around the nucleus in the same way as the planets revolving around the sun but he took a bold step and applied Planks’ quantum theory to the electrons revolving around the nucleus. He made postulates that give the idea about the energy level of the electron which are the orbits of the fixed energy. The theory also emphasized the quantization of angular momentum.

Complete answer:
The postulates of Bohr's theory are as follows:
1) The electrons in an atom revolve around the nucleus only in certain selected circular orbits. As long as the electrons remain in a particular orbit, it neither loses or gains energy. This leads to the idea that each orbit is associated with the definite energy level .i.e. definite whole number quanta of energy. These orbits are also known as energy shells or energy levels. Bohr named them as 1, 2, 3, 4, etc. Which are in turn associated with the principal quantum number K, L, M, N, etc.
2) The energy of the electron cannot change continuously. The change in the energy is observed only when the electrons jump from one energy level to another (Can be excitation or de-excitation).
3) The angular momentum of an electron around the nucleus is quantised. An electron, like any other body moving in a circular orbit, has an angular momentum given by the $\text{ m}\times \text{u}\times \text{r }$ ,where m is the mass, u is the linear velocity of an electron and r is the radius of the orbit. Bohr postulated that the angular momentum of an electron is given as follows,
$\text{ Angular momentum = mur = n }\dfrac{\text{h}}{\text{2 }\!\!\pi\!\!\text{ }}\text{ }$
Where n is an integer (1, 2, 3, etc.). Thus, the angular momentum of an electron may be a simple whole-number multiple of$\dfrac{\text{h}}{\text{2 }\!\!\pi\!\!\text{ }}\text{ }$. This is known as the quantization of angular momentum.

Note:
The Bohr's model of an atom has certain limitations.
1. The theory failed to explain the Zeeman Effect .It is the effect of the magnetic field on the spectra of atoms.
2. It failed to explain the stark effect. It is the effect of an electric field on the spectra of atoms.
3. Bohr was unable to explain the spectra for larger atoms.