Bohr model of atom is valid for :

The correct answer is Option (3) → Only one electron atoms and ions

Bohr's model considers a single electron moving round the nucleus under the Coulomb attraction alone. It therefore applies to every hydrogen-like (one-electron) system: the hydrogen atom and ions such as \( \mathrm{He^{+}} \) and \( \mathrm{Li^{2+}} \). It cannot handle the electron-electron interactions of atoms with two or more electrons.

Option (1) Only hydrogen atom — Incorrect. The model also works for one-electron ions such as \( \mathrm{He^{+}} \) and \( \mathrm{Li^{2+}} \), so "only hydrogen" is too narrow.

Option (2) Only one electron atom — Incorrect. The hydrogen atom is the only neutral one-electron atom; the option leaves out the one-electron ions.

Option (3) Only one electron atoms and ions — Correct. It covers the hydrogen atom and all hydrogen-like ions.

Option (4) All the atoms — Incorrect. Many-electron atoms are not described by the Bohr model.

NCERT (2026-27):
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.”

De Broglie’s Explanation of Bohr’s Second Postulate of Quantisation – “… Hydrogenic atoms are the atoms consisting of a nucleus with positive charge +Ze and a single electron, where Z is the proton number. Examples are hydrogen atom, singly ionised helium, doubly ionised lithium, and so forth.”