We are introduced to the Bohr model of atom one time or the other in the course of physics. This model has its place in the history of quantum mechanics and particularly in explaining the structure of an atom. It has become a milestone since Bohr introduced the revolutionary idea of definite energy orbits for the electrons, contrary to the classical picture requiring an accelerating particle to radiate. Bohr also introduced the idea of quantisation of angular momentum of electrons moving in definite orbits. Thus, it was a semi-classical picture of the structure of atom.
Now with the development of quantum mechanics, we have a better understanding of the structure of atom. Solutions of the Schrodinger wave equation assign a wave-like description to the electrons bound in an atom due to attractive forces of the protons.
An orbit of the electron in the Bohr model is the circular path of motion of an electron around the nucleus.
- Bohr model is valid only for one-electron atom/ions; an energy value, assigned to each orbit, depends on the principal quantum number \( n \) in this model. We know that energy associated with a stationary state of an electron depends on \( n \) only. For one-electron atoms/ions. For a multi-electron atom/ion, this is not true.
Bohr's quantisation conditions:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Angular momentum is quantised
Bohr's quantisation condition (his second postulate) states that the electron can revolve only in those orbits for which its angular momentum is an integral multiple of \( h/2\pi \):
\( \displaystyle L = m v r = \frac{n h}{2\pi}, \quad n = 1, 2, 3, \ldots \)
The passage also says that Bohr introduced the idea of quantisation of angular momentum of electrons moving in definite orbits.
Option (1) Charge is quantised — Incorrect. Quantisation of charge is a separate property of charge; it plays no part in Bohr's condition.
Option (2) Angular momentum is quantised — Correct. This is Bohr's quantisation condition, \( L = nh/2\pi \).
Option (3) Circumference of electron orbit is quantised — Incorrect. Bohr did not postulate this; \( 2\pi r = n\lambda \) is de Broglie's later interpretation of the angular momentum condition.
Option (4) Energy is quantised — Incorrect. Discrete energy levels follow as a result of the angular momentum condition; they are not the quantisation condition itself.
NCERT (2026-27):
Bohr Model of the Hydrogen Atom – “… (ii) Bohr’s second 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 h/2π where h is the Planck’s constant (= 6.6 × 10–34 J s). Thus the angular momentum (L) of the orbiting electron is quantised.”