Physics · Atoms · NEET
1) STATIONARY ORBITS: an electron can revolve in certain stable orbits without giving off any radiation, even though it is accelerating. 2) QUANTISATION: only orbits where the angular momentum L = mvr equals a whole-number multiple of h/2pi are allowed, so mvr = nh/2pi with n = 1, 2, 3... 3) FREQUENCY CONDITION: the atom emits or absorbs energy only when the electron jumps from one allowed orbit to another, and the photon energy is hf = E(initial) - E(final). Remember 'Stay, Step, Jump'.
Classical physics says any accelerating charge must radiate energy and spiral into the nucleus. Bohr simply POSTULATED (assumed as a new rule) that this classical law does not apply inside the atom. In a stationary orbit the electron does not lose energy, so the atom is stable. Bohr gave no deeper reason; he treated it as a fresh quantum assumption. This is why it is called a postulate and not a derivation.
No. 'Stationary' does NOT mean at rest. The electron is still moving and revolving fast. 'Stationary' means the orbit has a fixed, unchanging energy - the state stays the same in time because no energy is radiated. So it is a state of constant energy, not a state of no motion. This word confuses many students in NEET.
m = mass of electron, v = its speed, r = orbit radius, so mvr is the angular momentum. h is Planck's constant and n is a positive whole number (1, 2, 3...) called the principal quantum number. The equation says angular momentum can only take the values h/2pi, 2h/2pi, 3h/2pi and so on - never a value in between. This 'quantisation' picks out which orbits are allowed.
Only during a JUMP between two allowed orbits, not while sitting in one orbit. When the electron falls from a higher-energy orbit (Ei) to a lower one (Ef), the atom emits one photon of energy hf = Ei - Ef. If it absorbs a photon of the right energy, it jumps up. Inside a single stationary orbit no light is emitted.
Rutherford's atom is unstable: the orbiting electron should radiate energy continuously, spiral inward, and produce a continuous spectrum. Bohr fixes this with three postulates - fixed non-radiating orbits, quantised angular momentum, and photon emission only on jumps. Bohr's model explains the stability of the atom and the sharp line spectrum of hydrogen, which Rutherford could not.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Three: (1) stationary non-radiating orbits, (2) quantisation of angular momentum mvr = nh/2pi, and (3) the frequency condition hf = Ei - Ef for emission or absorption.
The second postulate: the angular momentum of the electron is quantised as mvr = nh/2pi, where n = 1, 2, 3... This directly leads to the formulas for orbit radius, velocity, and energy.
No. It works well for hydrogen and hydrogen-like single-electron ions such as He+ and Li2+. It fails for atoms with two or more electrons and cannot explain fine spectral details, which is a key limitation.
h = 6.63 x 10^-34 J s (Planck's constant), so h/2pi is about 1.05 x 10^-34 J s. This quantity is often written as h-bar and is the smallest step of angular momentum.
Yes. Classical physics says an accelerating electron must radiate and fall into the nucleus. Bohr's first postulate deliberately breaks that rule for atomic-scale motion, marking a radical departure from classical mechanics.
