50. Which of the following phenomena is explained by Bohr’s model?
[RRB Group D — 14 Aug 2026, Shift 1]
(A) Rutherford scattering
(B) Diffraction of neutron
(C) Diffraction of proton
(D) Hydrogen line spectrum
Answer: (D) Hydrogen line spectrum
Explanation: Bohr proposed discrete, stable electron energy levels in hydrogen. Electrons emit or absorb photons while moving between levels, producing specific spectral lines rather than a continuous spectrum. This successfully explains the regular pattern of the hydrogen line spectrum.
49. Which of the following best represents the characteristic properties of isotopes?
[RRB Group D — 14 Aug 2026, Shift 3]
(A) Both chemical and physical properties of isotopes are similar.
(B) Both chemical and physical properties of isotopes are different.
(C) Chemical properties are similar, but physical properties are different.
(D) Chemical properties are different, but physical properties are similar.
Answer: (C) Chemical properties are similar, but physical properties are different.
Explanation: Isotopes contain equal numbers of protons and, when neutral, electrons, giving them very similar chemical behaviour. Their neutron counts and masses differ, so mass-dependent physical properties can differ. Hydrogen and deuterium illustrate these isotope effects especially clearly.
48. Which of the following statements is true regarding isotopes?
[RRB Group D — 12 Aug 2026, Shift 1]
(A) Isotopes are different elements with the same atomic number.
(B) Isotopes have different atomic numbers but the same mass number.
(C) Isotopes have the same number of neutrons.
(D) Isotopes have the same number of protons but different numbers of neutrons.
Answer: (D) Isotopes have the same number of protons but different numbers of neutrons.
Explanation: Atomic number equals proton count and uniquely identifies an element. Isotopes therefore keep their proton count unchanged but possess different neutron numbers, leading to different mass numbers. Carbon-12 and carbon-14, for instance, each contain six protons.
47. Which statements are correct according to Bohr’s model of the atom?
(1) Electrons revolve in fixed circular energy levels.
(2) Electron energy changes continuously in one orbit.
(3) Energy is absorbed or emitted when an electron changes levels.
[RRB Group D — 12 Aug 2026, Shift 3]
(A) All statements are correct.
(B) Only 1 and 3 are correct.
(C) Only 1 and 2 are correct.
(D) Only 2 and 3 are correct.
Answer: (B) Only 1 and 3 are correct.
Explanation: In Bohr’s model an electron retains a definite energy in each permitted stationary orbit; it does not continually lose energy there. Radiation is involved only in transitions between orbits. Hence statements 1 and 3 are true, but 2 is false.
46. The maximum number of electrons present in a shell is given by which formula? (Here, n is the shell number.)
[RRB Group D — 11 Aug 2026, Shift 1]
(A) 2n
(B) 2n³
(C) 2n⁻²
(D) 2n²
Answer: (D) 2n²
Explanation: A shell with principal quantum number n contains n² spatial orbitals. Each orbital accommodates at most two electrons with opposite spins, giving 2n² electrons per shell. Consequently, the K, L and M shells can hold 2, 8 and 18 electrons, respectively.
45. According to Bohr’s atomic model, which statement is incorrect regarding discrete electron orbits?
[RRB Group D — 9 Aug 2026, Shift 1]
(A) They are specific permitted orbits within the atom.
(B) Electrons radiate energy exponentially while revolving in them.
(C) Electrons do not radiate energy while revolving in them.
(D) These orbits are also called energy levels.
Answer: (B) Electrons radiate energy exponentially while revolving in them.
Explanation: Bohr’s postulates designate allowed orbits as stationary states. An electron occupying one of these states does not continuously radiate electromagnetic energy. Emission occurs during a downward transition, while an upward transition requires absorption of energy of the proper amount.
44. Why do elements with the same atomic number exhibit nearly identical chemical properties even when their mass numbers differ?
[RRB Group D — 6 Aug 2026, Shift 1]
(A) They have similar atomic masses.
(B) They contain the same number of neutrons.
(C) They have the same numbers of protons and electrons.
(D) They occur in the same physical state.
Answer: (C) They have the same numbers of protons and electrons.
Explanation: Same atomic number means the atoms are isotopes of one element. Neutral isotopes also have identical electron counts and electronic configurations, especially identical valence electrons. Since bonding is controlled chiefly by electrons, their chemical properties remain very similar despite differing neutron counts.
43. Which subatomic particle was discovered by J. Chadwick in 1932?
[SSC MTS — 11 Feb 2026, Shift 1 (2025 cycle)]
(A) Proton
(B) Electron
(C) Neutron
(D) Alpha particle
Answer: (C) Neutron
Explanation: James Chadwick demonstrated the neutron in 1932 through experiments involving radiation produced by alpha-particle bombardment of beryllium. Neutrons have no net electrical charge and reside within atomic nuclei. The discovery clarified isotopic masses and helped explain nuclear structure.
42. According to Bohr’s model of the atom, in which orbits can electrons revolve around the nucleus?
[RRB Group D — 10 Feb 2026, Shift 2]
(A) Any arbitrary circular path
(B) Elliptical orbits of variable energy
(C) Spiral paths continuously losing energy
(D) Certain fixed circular paths called discrete orbits
Answer: (D) Certain fixed circular paths called discrete orbits
Explanation: Bohr rejected unrestricted classical electron paths and introduced only certain allowed circular orbits, each corresponding to a definite energy. This quantization accounts for atomic stability in his model. An electron changes energy by jumping between these permitted levels.
41. Isotopes of the same element differ in which of the following?
[MP Police SI — 21 Jan 2026, Shift 1]
(A) Number of protons
(B) Atomic number
(C) Number of neutrons
(D) Chemical properties
Answer: (C) Number of neutrons
Explanation: All isotopes of an element have an identical proton count, so their atomic number remains fixed. The differing number of neutrons changes the mass number. For example, hydrogen-1 has no neutron, while hydrogen-2 has one neutron.