201. Bohr’s atomic model was introduced mainly to keep Rutherford’s nuclear idea while adding rules that explain
ⓐ. atomic stability and hydrogen line spectra
ⓑ. the chemical taste of elements
ⓒ. the disappearance of electrons from atoms
ⓓ. alpha-particle detection on a zinc sulphide screen only
Correct Answer: atomic stability and hydrogen line spectra
Explanation: Rutherford’s model successfully explained the small positive nucleus and the mostly empty atom. However, it could not explain why revolving electrons do not collapse into the nucleus. It also could not explain why hydrogen gives sharp spectral lines. Bohr’s model kept the central nucleus but added quantum postulates about allowed orbits and transitions. These postulates were designed to explain stability and the hydrogen spectrum together.
202. Bohr’s model is most directly successful for
ⓐ. one-electron atoms and ions
ⓑ. all multi-electron atoms with complete accuracy
ⓒ. molecules with band spectra only
ⓓ. alpha particles passing through gold foil only
Correct Answer: one-electron atoms and ions
Explanation: Bohr’s model works best for systems containing only one electron. Hydrogen has one electron, and ions such as \(He^+\) and \(Li^{2+}\) also have one electron. In such systems, the electron mainly interacts with a single nucleus, making the model mathematically simple. Multi-electron atoms involve electron-electron interactions that Bohr’s simple model cannot handle fully. The scope of the model is therefore hydrogen and hydrogen-like species, not all atoms equally.
203. A hydrogen-like ion means an ion that
ⓐ. has no nucleus
ⓑ. has exactly one neutron and no proton
ⓒ. gives only continuous spectra
ⓓ. has only one electron
Correct Answer: has only one electron
Explanation: A hydrogen-like ion is similar to hydrogen in having just one electron. The nucleus may have charge \(+Ze\), where \(Z\) can be greater than \(1\). Examples include \(He^+\), \(Li^{2+}\), and \(Be^{3+}\). The key feature is not that the nucleus is identical to hydrogen, but that only one electron is present. This is why Bohr’s formulas can be extended from hydrogen to hydrogen-like ions by including \(Z\).
204. A table compares Rutherford’s model and Bohr’s model.
| Row | Feature | Suitable statement |
|---|
| P | Rutherford model | Explained the nuclear structure from scattering |
| Q | Rutherford model | Could not explain atomic stability classically |
| R | Bohr model | Introduced stationary orbits |
| S | Bohr model | Rejected the existence of the nucleus |
The row that needs correction is
ⓐ. Row P
ⓑ. Row S
ⓒ. Row Q
ⓓ. Row R
Correct Answer: Row S
Explanation: Rutherford’s model explained alpha-particle scattering by introducing a tiny positive nucleus. It also faced the stability problem because a revolving charged electron should radiate energy in classical theory. Bohr’s model retained the Rutherford nucleus instead of rejecting it. Its new idea was that only certain stationary orbits are allowed. Row S is wrong because Bohr’s model combines the nuclear picture with quantum postulates.
205. The main reason Bohr’s model was not just a repetition of Rutherford’s model is that Bohr added
ⓐ. a uniformly spread positive charge throughout the atom
ⓑ. quantised electron motion and radiation
ⓒ. the idea that electrons are absent from atoms
ⓓ. the idea that alpha particles are neutral
Correct Answer: quantised electron motion and radiation
Explanation: Rutherford’s model had a central nucleus and revolving electrons. Bohr kept this broad picture but added new restrictions that were not present in the classical model. He proposed that electrons can revolve only in certain allowed orbits without radiating energy. He also connected radiation with transitions between these orbits. These quantum restrictions are what allowed Bohr’s model to address stability and spectral lines.
206. Consider the following statements about Bohr’s model.
I. It was designed to explain the stability of atoms.
II. It explained the hydrogen spectrum using quantised energy changes.
III. It is a complete model for all fine details of multi-electron atoms.
ⓐ. II and III only
ⓑ. I and III only
ⓒ. I, II, and III
ⓓ. I and II only
Correct Answer: I and II only
Explanation: Bohr’s model introduced stationary orbits to explain why the electron does not continuously radiate energy while in an allowed orbit. It also used energy differences between orbits to explain hydrogen spectral lines. These two achievements made it a major improvement over Rutherford’s model. However, Bohr’s model is not a complete theory for multi-electron atoms or fine spectral details. Its strongest success is with hydrogen and hydrogen-like one-electron systems.
207. Assertion: Bohr’s model kept the central nucleus of Rutherford’s model.
Reason: Bohr’s model was built by adding quantum postulates to the nuclear atom.
ⓐ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓑ. Assertion is true, but Reason is false
ⓒ. Assertion is false, but Reason is true
ⓓ. Both Assertion and Reason are true, and Reason explains Assertion
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: Bohr did not return to Thomson’s spread-out positive charge model. He accepted Rutherford’s conclusion that the atom has a small positive nucleus. The new part of Bohr’s model was the introduction of quantum postulates about allowed orbits and radiation during transitions. These postulates were added to the nuclear atom to solve problems of stability and spectra. The Reason correctly explains why the central nucleus remains in Bohr’s model.
208. A claim says, “Bohr’s model is useful only because it explains alpha-particle backscattering.” The better statement is that Bohr’s model mainly explains
ⓐ. why gold foil can be made thin
ⓑ. why zinc sulphide screens glow
ⓒ. atomic stability and hydrogen spectral lines
ⓓ. why alpha particles are produced by radioactive sources
Correct Answer: atomic stability and hydrogen spectral lines
Explanation: Alpha-particle backscattering was explained by Rutherford’s nuclear model. Bohr’s model used the nuclear picture but was introduced to solve different problems. It explained why electrons in allowed orbits do not radiate continuously. It also connected spectral lines with transitions between allowed energy states. The scattering experiment led to the nuclear atom, while Bohr’s postulates addressed stability and spectra.
209. In Bohr’s model, a stationary orbit is an orbit in which the electron
ⓐ. revolves without radiating energy
ⓑ. remains fixed at one point in space
ⓒ. moves in a straight line away from the nucleus
ⓓ. loses energy continuously and spirals inward
Correct Answer: revolves without radiating energy
Explanation: The word stationary in Bohr’s model does not mean that the electron is at rest. It means that the electron can revolve in an allowed orbit without emitting radiation. The energy of the electron remains constant while it stays in that orbit. This postulate directly avoids the classical collapse predicted for Rutherford’s revolving electron. Radiation occurs only during transitions between stationary states, not during motion in a permitted orbit.
210. The phrase “stationary state” in Bohr’s model refers most closely to
ⓐ. a state where the electron has no motion at all
ⓑ. a state where the nucleus has no charge
ⓒ. a state where all wavelengths are emitted continuously
ⓓ. a definite-energy non-radiating state
Correct Answer: a definite-energy non-radiating state
Explanation: A stationary state is an allowed state of the atom with a definite energy. The electron may be moving in a circular orbit in the Bohr picture, but it does not radiate while it remains in that allowed state. This idea was introduced to explain atomic stability. It is different from the classical expectation that any revolving charged electron should radiate. The term stationary describes the constancy of the state’s energy, not absence of electron motion.
211. Bohr’s first postulate about stationary orbits was needed because classical theory predicted that a revolving electron should
ⓐ. radiate energy continuously
ⓑ. absorb all photons at every wavelength
ⓒ. become a neutron
ⓓ. remain stable without any special rule
Correct Answer: radiate energy continuously
Explanation: In classical theory, a charged particle undergoing acceleration radiates energy. A revolving electron is accelerated because its velocity direction keeps changing. Rutherford’s model therefore predicted continuous radiation and inward spiralling of the electron. Bohr’s stationary-orbit postulate stopped this collapse by allowing special non-radiating orbits. The postulate was not a small correction; it changed the rule for radiation inside allowed atomic states.
212. A passage describes an electron in a Bohr atom.
An electron is in one of the allowed circular orbits around the nucleus. It remains in that orbit for some time without changing its energy.
According to Bohr’s postulate, during this time the electron
ⓐ. emits a continuous spectrum
ⓑ. falls gradually into the nucleus
ⓒ. emits no radiation
ⓓ. changes its charge from \(-e\) to \(+e\)
Correct Answer: emits no radiation
Explanation: Bohr’s model permits electrons to revolve in certain allowed orbits without radiating energy. While the electron remains in such an orbit, its energy is constant. This is the meaning of a stationary orbit or stationary state. Radiation is associated with a transition between two allowed states, not with the electron simply staying in one state. This rule is what makes Bohr’s atom stable against the classical collapse problem.
213. A table gives possible descriptions of electron motion in Bohr’s model.
| Row | Description | Bohr-model status |
|---|
| P | Electron in an allowed orbit | No radiation while it remains there |
| Q | Electron changes from one allowed orbit to another | Radiation may be emitted or absorbed |
| R | Electron in a stationary state | Energy remains constant |
| S | Electron in every possible circular orbit | All orbits are allowed |
The row that conflicts with Bohr’s model is
ⓐ. Row P
ⓑ. Row Q
ⓒ. Row S
ⓓ. Row R
Correct Answer: Row S
Explanation: Bohr’s model allows only certain circular orbits, not every possible orbit. In an allowed orbit, the electron does not radiate while it remains there. When the electron changes from one allowed orbit to another, radiation can be emitted or absorbed depending on the direction of the transition. A stationary state has constant energy. Row S conflicts with quantisation because it treats the allowed orbits as continuous instead of restricted.
214. The stability of Bohr’s atom depends most directly on the idea that
ⓐ. electrons are not charged particles
ⓑ. the nucleus has no positive charge
ⓒ. atoms contain no moving particles
ⓓ. stationary orbits do not radiate
Correct Answer: stationary orbits do not radiate
Explanation: If an electron continuously lost energy while revolving, it would spiral into the nucleus. Bohr avoided this by postulating that an electron in a stationary orbit does not radiate. Since no energy is lost in that allowed orbit, the electron does not collapse into the nucleus. This directly explains atomic stability at the level of Bohr’s model. The electron is still charged and moving; the special point is the non-radiating nature of allowed orbits.
215. A claim states, “Since Bohr’s electron revolves, it must radiate continuously just as in Rutherford’s model.” The Bohr-model correction is that
ⓐ. the electron stops moving in every allowed orbit
ⓑ. the electron becomes electrically neutral in a stationary orbit
ⓒ. allowed stationary orbits do not radiate
ⓓ. the nucleus disappears during electron revolution
Correct Answer: allowed stationary orbits do not radiate
Explanation: Bohr’s model deliberately modifies the classical radiation expectation for atomic orbits. In a permitted stationary orbit, the electron can revolve without emitting radiation. This is not the same as saying the electron has no motion. It means the classical rule of continuous radiation is not applied to allowed stationary states. Radiation is produced only when the electron jumps between allowed states.
216. In Bohr’s model, radiation is emitted when the electron
ⓐ. drops from a higher to a lower stationary state
ⓑ. remains in the same stationary orbit
ⓒ. moves from a lower energy state to a higher energy state by absorbing energy
ⓓ. stays at rest inside the nucleus
Correct Answer: drops from a higher to a lower stationary state
Explanation: A photon is emitted when the atom loses energy. In Bohr’s model, this happens when the electron makes a transition from a higher energy stationary state to a lower energy stationary state. The energy difference appears as photon energy. While the electron remains in the same stationary state, no radiation is emitted. Upward transitions require absorption of energy rather than emission.
217. The energy of a photon emitted in a Bohr transition depends on
ⓐ. the orbital frequency of the electron alone
ⓑ. the difference between the two allowed energies
ⓒ. the thickness of the gold foil
ⓓ. the number of scintillations on a zinc sulphide screen
Correct Answer: the difference between the two allowed energies
Explanation: Bohr’s frequency condition connects emitted radiation with energy differences between stationary states. If the electron moves from a higher state to a lower state, the photon energy equals the difference between those two energies. This is written as \(h\nu=E_i-E_f\) for emission, with \(E_i\gt E_f\). The frequency of the emitted photon is therefore fixed by the level separation. It is not determined by the alpha-scattering apparatus or by continuous radiation during one orbit.
218. For an emission transition in Bohr’s model, \(E_i\) is the initial higher energy and \(E_f\) is the final lower energy. The frequency condition is
ⓐ. \(h\nu=E_f-E_i\)
ⓑ. \(h\nu=E_i-E_f\)
ⓒ. \(\nu=h(E_i+E_f)\)
ⓓ. \(h\nu=\frac{E_f}{E_i}\)
Correct Answer: \(h\nu=E_i-E_f\)
Explanation: \( \textbf{Transition type:} \) The question describes emission.
\( \textbf{Energy order:} \) For emission, \(E_i\gt E_f\).
\( \textbf{Photon energy:} \)
\[
E_{\text{photon}}=h\nu
\]
\( \textbf{Energy lost by atom:} \)
\[
\Delta E=E_i-E_f
\]
\( \textbf{Frequency condition:} \)
\[
h\nu=E_i-E_f
\]
\( \textbf{Sign check:} \) The right side is positive because the atom moves from higher energy to lower energy.
\( \textbf{Final answer:} \) The emission frequency condition is \(h\nu=E_i-E_f\).
219. For absorption in Bohr’s model, the electron moves upward from energy \(E_i\) to energy \(E_f\), where \(E_f\gt E_i\). The absorbed photon must satisfy
ⓐ. \(h\nu=E_i-E_f\)
ⓑ. \(h\nu=0\)
ⓒ. \(h\nu=E_f-E_i\)
ⓓ. \(h\nu=E_iE_f\)
Correct Answer: \(h\nu=E_f-E_i\)
Explanation: \( \textbf{Transition type:} \) Absorption raises the atom to a higher energy state.
\( \textbf{Energy order:} \) \(E_f\gt E_i\).
\( \textbf{Energy gained by atom:} \)
\[
\Delta E=E_f-E_i
\]
\( \textbf{Photon relation:} \)
\[
E_{\text{photon}}=h\nu
\]
\( \textbf{Absorption condition:} \)
\[
h\nu=E_f-E_i
\]
\( \textbf{Physical meaning:} \) The photon must supply exactly the allowed energy gap.
\( \textbf{Final answer:} \) The absorbed photon energy is \(E_f-E_i\).
220. A graph description is given below.
Horizontal energy levels are drawn for a Bohr atom. An arrow points downward from a higher level to a lower level.
The arrow represents
ⓐ. absorption of a photon with zero energy
ⓑ. photon emission equal to the level gap
ⓒ. continuous radiation from an unchanged orbit
ⓓ. disappearance of the nucleus
Correct Answer: photon emission equal to the level gap
Explanation: In an energy-level diagram, a downward arrow means the atom moves from a higher energy state to a lower energy state. The atom loses energy in this process. The lost energy appears as an emitted photon. The photon energy is equal to the difference between the two levels, so \(h\nu=E_i-E_f\). The arrow does not represent radiation from an unchanged orbit; it represents a transition between stationary states.
