Q.State the postulates of Bohr's atomic model.

[!TLDR] Bohr's three postulates: (1) electrons move in circular orbits held by Coulomb attraction, (2) only orbits with angular momentum $=nh/2\pi$ are allowed and don't radiate, (3) a transition between orbits emits/absorbs a photon of energy equal to the orbit-energy difference. [!ANSWER] Circular orbits under electrostatic attraction; quantized angular momentum $m_evr=nh/2\pi$ defining non-radiating stable orbits; and a photon of energy $E_m-E_n$ emitted on a transition between orbits.

Bohr's atomic model rests on three postulates. First, electrons revolve around the nucleus in circular orbits, with the centripetal force required for this circular motion supplied entirely by the electrostatic force of attraction between the (negative) electron and the (positive) nucleus -- this part is identical to Rutherford's earlier model. Second, and this is Bohr's genuinely new contribution, the radius of an electron's orbit cannot take any arbitrary value: only orbits in which the electron's angular momentum equals an exact integer multiple of $h/2\pi$ are permitted, $m_evr=\frac{nh}{2\pi}$ (n = 1, 2, 3, ..., the principal quantum number); electrons occupying these special, quantized orbits are said to be in stable (or stationary) states and, contrary to classical electromagnetic theory, they do NOT radiate energy while remaining in such an orbit. Third, an electron can make a transition (a 'jump') from one allowed orbit to another of different energy; if it moves to a LOWER-energy orbit, the atom emits a single photon whose energy exactly equals the energy difference between the initial and final orbits, and conversely absorbing a photon of the right energy can drive the electron up to a higher orbit. [!ANSWER] Circular orbits under electrostatic attraction; quantized angular momentum $m_evr=nh/2\pi$ defining non-radiating stable orbits; and a photon of energy $E_m-E_n$ emitted on a transition between orbits.

Direct recall of the three postulates as stated in the model's construction.

Omitting the crucial 'do not radiate while in a stable orbit' clause of the second postulate, which is precisely the point that rescues the model from Rutherford's classical-collapse problem.