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 model of atom is:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Semi-classical picture of atomic structure
Bohr's model mixes two kinds of ideas. The electron moves in a definite circular orbit under the Coulomb force, exactly as in classical mechanics. On top of this, Bohr added quantum conditions: only orbits with angular momentum \( L = nh/2\pi \) are allowed, and the electron in such an orbit does not radiate.
Because it combines classical orbits with quantisation rules, the model is a semi-classical picture, as the passage itself states.
Option (1) Classical picture of atomic structure — Incorrect. Classical physics alone would make the orbiting electron radiate and spiral into the nucleus; Bohr's quantisation rules are not classical.
Option (3) Quantum picture of atomic structure — Incorrect. The full quantum picture comes from the Schrodinger equation, which gives a wave-like description of the electron with no definite orbit.
Option (4) Standing wave picture of atomic structure — Incorrect. The standing-wave idea for the electron is de Broglie's later explanation of Bohr's condition; it is not what Bohr's model itself is.
NCERT (2026-27):
Points to Ponder – “… 7. Bohr’s semiclassical model based on some aspects of classical physics and some aspects of modern physics also does not provide a true picture of the simplest hydrogenic atoms.”