Bohr's FIRST postulate of the hydrogen atom states that
Athe electron can revolve only in certain stable orbits WITHOUT emitting radiation
Bevery accelerating charged particle must emit electromagnetic radiation
Cthe energy of a photon is $h\nu$
Dthe angular momentum of the electron is continuous
Answer & Solution
Correct answer: A. the electron can revolve only in certain stable orbits WITHOUT emitting radiation
1. NCERT §12.4 lists three Bohr postulates explicitly.
2. First postulate: an electron in an atom can revolve in certain STABLE orbits WITHOUT the emission of radiant energy — contrary to what classical electromagnetism predicts.
3. These privileged orbits are called STATIONARY STATES; each has a definite total energy.
4. Option B is the CLASSICAL electromagnetic prediction, which Bohr's first postulate explicitly contradicts. Option C is Planck/Einstein's quantum hypothesis (not a Bohr postulate). Option D is the OPPOSITE of Bohr's second postulate (which says angular momentum is quantised).
_Source: NCERT Class 12 Physics Part 2, Ch 12 "Atoms", §12.4 (Bohr Model — first postulate), p. 8–9._
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