Assertion (A): According to Bohr, the angular momentum of the electron revolving around the nucleus is quantised.
Reason (R): Each electron interacts not only with the positively charged nucleus but also with all the other electrons.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
(A) According to Bohr, the angular momentum of the electron revolving around the nucleus is quantised. — True. This is Bohr’s second postulate: the electron revolves only in those orbits for which \( L=nh/2\pi \), with \( n=1,2,3,\ldots \).
(R) Each electron interacts not only with the positively charged nucleus but also with all the other electrons. — True. In an atom with more than one electron, each electron is attracted by the nucleus and also repelled by every other electron. This is why the Bohr model, which includes only the electron–nucleus force, cannot be extended even to helium.
Why R does not explain A?
R is about electron–electron interactions in atoms with many electrons. The Bohr model leaves these out, so R only points to a limitation of the model. It says nothing about why angular momentum is quantised.
Quantisation of angular momentum was a postulate of Bohr. De Broglie explained it by the wave nature of the electron. Only orbits that hold a whole number of wavelengths, \( 2\pi r_n=n\lambda \) with \( \lambda=h/mv_n \), can persist. This gives \( mv_nr_n=nh/2\pi \).
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.”
De Broglie’s Explanation of Bohr’s Second Postulate of Quantisation – “(i) The Bohr model is applicable to hydrogenic atoms. It cannot be extended even to mere two electron atoms such as helium. The analysis of atoms with more than one electron was attempted on the lines of Bohr’s model for hydrogenic atoms but did not meet with any success. Difficulty lies in the fact that each electron interacts not only with the positively charged nucleus but also with all other electrons.”
De Broglie’s Explanation of Bohr’s Second Postulate of Quantisation – “… Thus de Broglie hypothesis provided an explanation for Bohr’s second postulate for the quantisation of angular momentum of the orbiting electron. The quantised electron orbits and energy states are due to the wave nature of the electron and only resonant standing waves can persist.”