Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): Atoms of most of the elements are stable and emit a characteristic spectrum.
Reason (R): The spectrum provides useful information about the atomic structure.
Choose the correct answer from the options given below.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A and R are both true but R is NOT the correct explanation of A.
Assertion (A) Atoms of most of the elements are stable and emit a characteristic spectrum. — True. Most atoms do not collapse. When excited, each element emits radiation of only certain wavelengths that belong to that element alone. This is a line spectrum. Hydrogen, for example, always gives the same set of lines.
Reason (R) The spectrum provides useful information about the atomic structure. — True. The lines of the spectrum come from transitions between the energy levels of the atom. So studying them shows how the atom is built. Bohr’s model was made to explain the hydrogen spectrum.
Why R does not explain A?
R states a use of the spectrum to the scientist. It says nothing about why the atom is stable or why it emits only certain wavelengths.
Both facts in A come from the structure of the atom itself. By Bohr’s first postulate the electron revolves in certain stable orbits without emitting radiant energy. Jumps between these definite states give photons of definite energy \( h\nu=E_i-E_f \). Hence the atom gives a characteristic line spectrum.
NCERT (2026-27):
Summary – “5. Atoms of most of the elements are stable and emit characteristic spectrum. The spectrum consists of a set of isolated parallel lines termed as line spectrum. It provides useful information about the atomic structure.”
Bohr Model of the Hydrogen Atom – “(i) Bohr’s first postulate was that an electron in an atom could revolve in certain stable orbits without the emission of radiant energy, contrary to the predictions of electromagnetic theory. …”