Which one of the following scientists mentioned that an electron in an atom could revolve in certain stable orbits without the emission of radiant energy?
- (a)Ernest Rutherford
- (b)J.J. Thomson
- (c)Niels Bohr
- (d)Albert Einstein
Correct — C, Niels Bohr. In his 1913 model of the atom, the Danish physicist Niels Bohr proposed that electrons move only in certain allowed, stationary orbits of fixed energy, and that while an electron stays in such an orbit it does not radiate energy — it emits or absorbs energy only when it jumps between orbits. This postulate resolved the problem in Rutherford's model, where an orbiting electron should have radiated energy continuously and spiralled into the nucleus. Hence the scientist is Bohr, option (c).
- (a)Ernest Rutherford — Rutherford's nuclear model (1911) put electrons orbiting a tiny dense nucleus, but by classical physics such electrons would radiate energy and collapse inward — it was precisely this failing that Bohr's stable-orbit idea fixed.
- (b)J.J. Thomson — Thomson discovered the electron and gave the 'plum pudding' model, with electrons embedded in a sphere of positive charge; he did not propose fixed, radiationless orbits.
- (d)Albert Einstein — Einstein explained the photoelectric effect and developed relativity, but the specific postulate of stable, non-radiating electron orbits belongs to Bohr, not Einstein.
Early atomic models evolved rapidly. Rutherford placed electrons around a nucleus but could not explain why they did not radiate energy and fall in. Bohr fixed this by postulating quantised, stable orbits: an electron in an allowed orbit has a definite energy and does not emit radiation, and light is given off or absorbed only during a transition between orbits, with energy equal to the difference between the two levels.
Match the idea to the scientist. 'Stable orbits with no radiation' is the signature of Bohr's 1913 model. Rutherford supplied the nucleus but not the stability, Thomson gave an earlier model, and Einstein worked on light quanta and relativity — so Bohr is the only fit.
- Niels Bohr proposed his model of the hydrogen atom in 1913.
- Electrons occupy only certain allowed orbits of fixed energy, called stationary states, and do not radiate while in them.
- Energy is emitted or absorbed only when an electron jumps between orbits, as a photon equal to the energy difference.
- Bohr's model resolved the instability of Rutherford's nuclear atom, and Bohr won the 1922 Nobel Prize in Physics.
- Crediting Rutherford (the nucleus) or Thomson (the electron and plum-pudding model) with the stable-orbit idea.
- Thinking electrons radiate continuously while orbiting — Bohr said they do not, within an allowed orbit.
- Forgetting that energy is emitted only when an electron moves between orbits, not while it stays in one.
A name-the-scientist recall — the 'stable orbits without radiating energy' postulate points to Bohr.
Regarding the atom of a chemical element, the magnetic quantum number refers to
- (a) orientation
- (b) shape
- (c) size
- (d) spin
Answer(a) orientation — the magnetic quantum number gives the orientation of an atomic orbital in space.
Extends the same theme of atomic structure — how electrons are arranged in an atom — from Bohr's stable orbits to the quantum numbers that describe the orbitals succeeding them.
- practice — not a real PYQ
According to Bohr's model, an atom emits a photon of light when an electron:
- (a)stays in its orbit
- (b)jumps from a higher to a lower energy orbit
- (c)is removed from the atom
- (d)collides with the nucleus
Answer(b) jumps from a higher to a lower energy orbit — the photon carries the energy difference between the levels.
- practice — not a real PYQ
The limitation of Rutherford's atomic model that Bohr's model addressed was that orbiting electrons should:
- (a)repel the nucleus
- (b)continuously radiate energy and spiral into the nucleus
- (c)never move at all
- (d)carry no electric charge
Answer(b) continuously radiate energy and spiral into the nucleus — Bohr's stable orbits removed this instability.