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.
40. Which one of the following is a radioactive isotope of carbon?
[SSC JE Mechanical — 6 Dec 2025, Shift 2]
(A) Carbon-12
(B) Carbon-13
(C) Carbon-14
(D) Carbon-6
Answer: (C) Carbon-14
Explanation: Carbon-14 contains six protons and eight neutrons. Unlike stable carbon-12 and carbon-13, it undergoes radioactive beta decay with a half-life of roughly 5,730 years. Its predictable decay underpins radiocarbon dating of once-living materials.
39. Which radioactive decay increases the atomic number of the parent nucleus by one without changing its mass number?
[SSC CGL — 24 Sept 2025, Shift 3]
(A) Alpha decay
(B) Beta-minus decay
(C) Beta-plus decay
(D) Gamma decay
Answer: (B) Beta-minus decay
Explanation: During beta-minus decay, a neutron converts into a proton, emitting an electron and an antineutrino. Proton count therefore rises by one, while the combined number of protons and neutrons stays constant. The daughter nucleus belongs to a different element.
38. In Bohr’s model of hydrogen, as electrons move to higher energy levels (larger n), how does the difference between successive energy levels change?
[SSC CGL — 22 Sept 2025, Shift 1]
(A) It increases continuously.
(B) It decreases continuously.
(C) It remains constant.
(D) It first increases, then decreases.
Answer: (B) It decreases continuously.
Explanation: The allowed hydrogen energies follow Eₙ = −13.6/n² electronvolts. Increasing n moves each level closer to zero, but the separation between consecutive levels becomes smaller. Thus transitions between neighbouring high-n states release lower-energy photons than transitions between nearby low-n states.
37. A major limitation of Rutherford’s atomic model was its inability to explain which of the following?
[SSC Selection Post (Higher Secondary) — 29 Aug 2025, Shift 2]
(A) The existence of isotopes
(B) The deflection of alpha particles
(C) The stability of the atom and the atomic spectrum
(D) The indivisible nature of atoms
Answer: (C) The stability of the atom and the atomic spectrum
Explanation: Under classical electromagnetic theory, an orbiting electron accelerates and should continuously radiate energy, eventually collapsing into the nucleus. Rutherford’s model also could not explain discrete atomic spectral lines. Bohr later introduced quantized stationary orbits to address these problems.
36. According to Bohr’s model, what happens when an electron goes from a higher-energy orbit to a lower-energy orbit?
[SSC Stenographer — 8 Aug 2025, Shift 1]
(A) Its angular momentum first becomes quantized.
(B) The electron gains energy.
(C) A photon equal to the energy difference is absorbed.
(D) A photon equal to the energy difference is emitted.
Answer: (D) A photon equal to the energy difference is emitted.
Explanation: A downward electronic transition releases the energy difference as a photon, whose frequency satisfies ΔE = hν. An upward transition instead requires photon absorption. Discrete energy gaps create the characteristic line spectrum of an atom rather than a continuous distribution.
35. Which observation in Rutherford’s experiment showed that an atom’s positive charge is concentrated in a tiny region?
[SSC Stenographer — 7 Aug 2025, Shift 1]
(A) Electrons occupied fixed energy levels.
(B) A very small fraction of alpha particles rebounded almost backwards.
(C) Most alpha particles passed straight through the foil.
(D) A small fraction of alpha particles deflected through small angles.
Answer: (B) A very small fraction of alpha particles rebounded almost backwards.
Explanation: A rare alpha particle rebounded nearly backwards because it approached a very small, massive, positively charged nucleus and experienced intense electrostatic repulsion. The more common straight-through trajectories showed that most of the atom’s volume is empty space.
34. Which one of the following statements is correct?
[UPSC NDA & NA (I) — 2025, General Ability Test]
(A) A neutron forms by combining an electron and a proton and is therefore neutral.
(B) An electron has a mass about 1/2000 that of a proton.
(C) An isotope of cobalt is used to treat goitre.
(D) J. J. Thomson proposed a nucleus containing only neutrons.
Answer: (B) An electron has a mass about 1/2000 that of a proton.
Explanation: An electron’s mass is approximately one eighteen-hundred-and-thirty-sixth that of a proton, commonly rounded to one two-thousandth. Cobalt-60 is used in radiotherapy, while iodine-131 treats certain thyroid conditions. Thomson proposed no atomic nucleus in his plum-pudding model.
33. In which year did Henry Moseley use X-rays to connect elemental spectra with atomic numbers?
[SSC GD Constable — 20 Feb 2025, Shift 3]
(A) 1925
(B) 1931
(C) 1913
(D) 1902
Answer: (C) 1913
Explanation: Moseley established a systematic relationship between characteristic X-ray frequencies and atomic number in 1913. This demonstrated that proton number, rather than average atomic mass, is fundamental to elemental identity and helped resolve anomalies in earlier periodic arrangements.
32. What was discovered through Rutherford’s gold foil experiment?
[RRB Technician Grade III — 29 Dec 2024, Shift 3]
(A) Neutron
(B) Nucleus
(C) Electron
(D) Proton
Answer: (B) Nucleus
Explanation: The unexpectedly large deflections of a few alpha particles indicated a tiny, dense, positively charged centre within an atom. Rutherford therefore established the nuclear model. Most particles passed through the foil, implying that much of the atomic volume is empty space.
31. Which of the following is NOT a subatomic particle?
[RRB Technician Grade III — 26 Dec 2024, Shift 1]
(A) Neutron
(B) Electron
(C) Proton
(D) Molecule
Answer: (D) Molecule
Explanation: Protons, neutrons and electrons are constituents of atoms. A molecule, however, is formed from two or more bonded atoms and is therefore a larger chemical entity, not a subatomic particle. Water and oxygen molecules provide familiar examples.
30. According to Rutherford’s model, where is most of an atom’s mass located?
[RRB Technician Grade III — 20 Dec 2024, Shift 2]
(A) In the electron shells
(B) In the nucleus
(C) In the electron cloud
(D) Equally throughout the atom
Answer: (B) In the nucleus
Explanation: Nearly the entire atomic mass is concentrated in protons and neutrons packed inside the very small nucleus. Electrons occupy much more space but have negligible mass by comparison. Rutherford inferred this dense core from alpha-particle scattering observations.
29. Which physicist is famous for the gold foil experiment?
[SSC Stenographer — 11 Dec 2024, Shift 2]
(A) John Dalton
(B) J. J. Thomson
(C) Ernest Rutherford
(D) Niels Bohr
Answer: (C) Ernest Rutherford
Explanation: Ernest Rutherford interpreted the alpha-particle scattering data collected by Geiger and Marsden, showing that atoms contain compact, positively charged nuclei. The experimental results challenged Thomson’s diffuse positive-charge model and led Rutherford to propose the nuclear atomic model.
28. Which statement is INCORRECT about Rutherford’s model of an atom?
[RRB ALP CBT-I — 26 Nov 2024, Shift 2]
(A) Electrons revolve around the nucleus in circular paths.
(B) The atom has a positively charged centre called the nucleus.
(C) Electrons revolve around the nucleus in elliptical paths.
(D) The nucleus is extremely small compared with the atom.
Answer: (C) Electrons revolve around the nucleus in elliptical paths.
Explanation: The textbook Rutherford model pictured electrons revolving around a small positively charged nucleus in circular paths, not specified elliptical paths. Its serious limitation was that classically accelerated electrons should radiate energy, making the proposed atom unstable without further assumptions.
27. Ernest Rutherford used gold foil about how many atoms thick for his alpha-particle scattering experiment?
[SSC CGL Tier-I — 11 Sept 2024, Shift 3]
(A) 500 atoms
(B) 1000 atoms
(C) 900 atoms
(D) 700 atoms
Answer: (B) 1000 atoms
Explanation: Gold is extremely malleable and can be beaten into very thin foil. A sheet roughly one thousand atoms thick allowed energetic alpha particles to interact with only a thin layer, making their scattering informative about the atom’s internal charge distribution.
26. Arrange these discoveries from earliest to latest: (I) Pi meson, (II) Neutron, (III) Electron, (IV) Proton.
[UPPSC RO/ARO General Studies — 11 Feb 2024]
(A) I, III, IV, II
(B) III, II, I, IV
(C) III, IV, II, I
(D) III, IV, I, II
Answer: (C) III, IV, II, I
Explanation: J. J. Thomson identified the electron in 1897. Rutherford’s proton work followed around 1917–1919, then Chadwick discovered the neutron in 1932. The pion was identified in cosmic-ray studies in 1947. Hence the sequence is III, IV, II, I.
25. Which Group 13 element has atomic number 113 and configuration [Rn] 5f¹⁴ 6d¹⁰ 7s² 7p¹?
[SSC CHSL Tier-I — 14 Aug 2023, Shift 1]
(A) Gallium
(B) Indium
(C) Nihonium
(D) Thallium
Answer: (C) Nihonium
Explanation: Nihonium, symbol Nh, is the synthetic element with atomic number 113. Its outer electronic arrangement ends in 7s²7p¹, consistent with Group 13. Gallium, indium and thallium are lighter members of the same group with different atomic numbers.
24. Which is the increasing order of electron affinity of the halogens?
[SSC CHSL Tier-I — 11 Aug 2023, Shift 1]
(A) F < I < Br < Cl
(B) I < Br < Cl < F
(C) I < Br < F < Cl
(D) I > Br > F < Cl
Answer: (C) I < Br < F < Cl
Explanation: For the halogens, chlorine has a greater first electron affinity than fluorine because fluorine’s compact 2p subshell produces greater electron-electron repulsion upon electron addition. Bromine and iodine show progressively lower values as atomic size and shielding increase.
23. In which year did Niels Bohr propose his quantum-based theory of the hydrogen atom?
[SSC CHSL Tier-I — 9 Aug 2023, Shift 1]
(A) 1919
(B) 1921
(C) 1909
(D) 1913
Answer: (D) 1913
Explanation: In 1913 Bohr developed a model that allowed electrons to occupy only specific stationary energy states. An electron could emit or absorb light by transitioning between them. This idea successfully accounted for regularities in the hydrogen spectrum.
22. Which set of quantum numbers can describe an electron in a 4f orbital?
[SSC CHSL Tier-I — 3 Aug 2023, Shift 1]
(A) n = 4, l = 3, m = +1, s = +½
(B) n = 3, l = 2, m = −2, s = +½
(C) n = 4, l = 4, m = −4, s = −½
(D) n = 4, l = 3, m = +4, s = +½
Answer: (A) n = 4, l = 3, m = +1, s = +½
Explanation: For a 4f electron the principal quantum number must be 4 and the orbital angular momentum quantum number must be l = 3. The magnetic quantum number may range from −3 to +3, and spin is ±½.
21. Whose work in 1808 provided a physical picture of compounds forming by combinations of different types of atoms?
[SSC CGL Tier-I — 26 Jul 2023, Shift 1]
(A) George Zweig
(B) J. J. Thomson
(C) Niels Bohr
(D) John Dalton
Answer: (D) John Dalton
Explanation: John Dalton’s atomic theory described matter as made of atoms and compounds as fixed combinations of atoms of different elements. His work helped connect the laws of chemical combination to a particle model, though later discoveries modified some of his assumptions.
20. Who argued in the early nineteenth century that each element consists of its own kind of atoms, differing from other elements in mass?
[SSC CGL Tier-I — 24 Jul 2023, Shift 1]
(A) Antoine Lavoisier
(B) John Dalton
(C) Amedeo Avogadro
(D) Robert Boyle
Answer: (B) John Dalton
Explanation: Dalton formulated an influential atomic theory in the early 1800s, proposing distinct atoms for different elements and explaining fixed composition in compounds. Modern science recognizes isotopes, so atoms of one element need not all have identical masses.
19. Which of the following elements has three valence electrons?
[RRB Group D — 7 Oct 2022, Shift 2]
(A) Al
(B) Ca
(C) N
(D) Mg
Answer: (A) Al
Explanation: Aluminium has atomic number 13 and the electronic configuration 2, 8, 3. Its outermost shell therefore contains three electrons. Calcium and magnesium each have two valence electrons, while nitrogen has five.
18. What mainly decides the valency of an atom?
[RRB Group D — 27 Sept 2022, Shift 1]
(A) The number of electrons in its outermost shell
(B) The number of electrons in its L shell
(C) The number of electrons in its innermost shell
(D) The number of electrons in its K shell
Answer: (A) The number of electrons in its outermost shell
Explanation: Valency reflects the combining capacity of an atom and is closely related to its outermost, or valence, electron configuration. Atoms may lose, gain or share electrons to approach stable configurations. Inner-shell electron totals do not generally decide ordinary valency.
17. Arrange B, Ne, K and O in increasing order of atomic number.
[RRB Group D — 27 Sept 2022, Shift 2]
(A) Ne, K, B, O
(B) K, B, O, Ne
(C) B, K, O, Ne
(D) B, O, Ne, K
Answer: (D) B, O, Ne, K
Explanation: The respective atomic numbers are boron 5, oxygen 8, neon 10 and potassium 19. Atomic number denotes the number of protons in an atomic nucleus. Ordering these proton counts from smallest to largest gives B, O, Ne, K.
16. According to Henry Moseley, which property is more fundamental to an element than its atomic mass?
[RRB Group D — 22 Sept 2022, Shift 2]
(A) Atomic number
(B) Atomic density
(C) Atomic radius
(D) Atomic size
Answer: (A) Atomic number
Explanation: Moseley’s X-ray experiments linked elemental identity and ordering to atomic number, later understood as nuclear proton count. Unlike average atomic mass, atomic number uniquely specifies an element. This principle also explains why isotopes share a position in the periodic table.
15. How does the number of valence electrons generally change while moving down a group of the periodic table?
[RRB Group D — 30 Aug 2022, Shift 2]
(A) It decreases.
(B) It increases.
(C) It first increases and then decreases.
(D) It remains constant.
Answer: (D) It remains constant.
Explanation: Elements in the same main-group column have comparable outer electronic configurations and hence typically the same number of valence electrons. Descending the group adds occupied shells, increasing atomic size, but the characteristic outer-electron count remains similar.
14. Which element has the greatest number of valence electrons among F, Al, H and N?
[RRB Group D — 22 Aug 2022, Shift 2]
(A) F
(B) Al
(C) H
(D) N
Answer: (A) F
Explanation: Fluorine has seven valence electrons; nitrogen has five, aluminium three and hydrogen one. The outermost electron count largely governs chemical bonding and reactivity. Fluorine typically gains or shares one electron to complete its valence octet.
13. The present atomic weight scale is based on which of the following?
[WBCS Mains 2021, Paper IV — held May 2022]
(A) Carbon-12
(B) Hydrogen-1
(C) Carbon-13
(D) Oxygen-16
Answer: (A) Carbon-12
Explanation: The unified atomic mass unit is defined as exactly one-twelfth the mass of a neutral carbon-12 atom in its ground state. Relative atomic masses are measured using this reference. Natural elemental atomic weights may be fractional because isotopes occur in mixtures.
12. The term “isotope” refers to which of the following?
[RRB NTPC CBT-I — 3 Apr 2021, Shift 1]
(A) Elements having the same bond structure
(B) Atoms with the same atomic number but different mass numbers
(C) Atoms with different atomic numbers but the same mass number
(D) Atoms with identical physical and chemical properties
Answer: (B) Atoms with the same atomic number but different mass numbers
Explanation: Isotopes share their nuclear proton number, which defines the element, but differ in neutron count and thus mass number. For example, chlorine-35 and chlorine-37 both have seventeen protons, yet possess eighteen and twenty neutrons respectively.
11. What are the atomic number and mass number, respectively, of tritium?
[RRB NTPC CBT-I — 13 Mar 2021, Shift 1]
(A) 1 and 3
(B) 3 and 1
(C) 1 and 2
(D) 1 and 1
Answer: (A) 1 and 3
Explanation: Tritium is hydrogen-3, containing one proton and two neutrons. Proton count gives atomic number 1; total nucleon count gives mass number 3. It is a radioactive hydrogen isotope, whereas protium has no neutron and deuterium has one.
10. The atomic number of an element is the number of which particles in the nucleus of each atom?
[RRB NTPC CBT-I — 4 Mar 2021, Shift 1]
(A) Protons
(B) Mesons
(C) Neutrons
(D) Electrons
Answer: (A) Protons
Explanation: Atomic number, denoted Z, counts the protons in the nucleus. For an electrically neutral atom this also equals the electron count, but an ion can have a different number of electrons. The proton count remains the defining feature of its element.
9. An isotope of which element is used in the treatment of cancer?
[UPSC NDA — 6 Sept 2020, General Ability Test]
(A) Uranium
(B) Cobalt
(C) Sodium
(D) Iodine
Answer: (B) Cobalt
Explanation: Cobalt-60 emits penetrating gamma radiation and has been used for external-beam cancer radiotherapy. Radioisotopes of iodine are commonly used for certain thyroid disorders instead. Medical use depends upon the kind, energy and location of the radiation emitted.
8. Who explained the phenomenon of the photoelectric effect?
[UPSC NDA — 21 Apr 2019, General Ability Test]
(A) Max Planck
(B) Albert Einstein
(C) Niels Bohr
(D) Ernest Rutherford
Answer: (B) Albert Einstein
Explanation: Einstein explained photoelectric emission by treating light as packets of energy, now called photons. An electron leaves a material only if the photon provides enough energy to overcome the work function. This established crucial evidence for light’s quantum behaviour.
7. What is the radius of an atom used to measure?
[RRB ALP CBT-I — 29 Aug 2018, Shift 1]
(A) Atomic number
(B) Atomic mass
(C) Atomic size
(D) Atomicity
Answer: (C) Atomic size
Explanation: Atomic radius serves as a measure of atomic size, although exact definitions vary for bonded and isolated atoms. Across a period it generally decreases as effective nuclear attraction rises; down a group it usually increases with additional electron shells.
6. The chemical properties of an element chiefly depend on which of the following?
[UPSC CDS General Knowledge — 5 Feb 2017]
(A) Its number of isotopes
(B) Its mass number
(C) Its total number of neutrons
(D) Its number of electrons in the outermost shell
Answer: (D) Its number of electrons in the outermost shell
Explanation: The outer-shell, or valence, electrons participate in ordinary chemical bonding and therefore strongly govern reactivity and compound formation. Isotope abundance and neutron counts can alter mass-dependent behaviour, but they do not usually change an element’s basic valence configuration.
5. Calcium (atomic number 20) and argon (atomic number 18) both have mass number 40. What are they called?
[UPSC CDS General Knowledge — 5 Feb 2017]
(A) Isotones
(B) Isochors
(C) Isobars
(D) Isotopes
Answer: (C) Isobars
Explanation: Isobars are atoms of different elements having identical mass numbers but different proton numbers. Calcium-40 contains twenty protons and twenty neutrons, while argon-40 contains eighteen protons and twenty-two neutrons. Their common nucleon total is forty.
4. Which one of the following statements is correct?
[UPSC NDA & NA (II) — 2015]
(A) Rutherford’s alpha-particle scattering experiment discovered the electron.
(B) J. J. Thomson suggested that the nucleus contains protons.
(C) The atomic number equals the number of protons in the atomic nucleus.
(D) The mass number equals the number of electrons in the shells.
Answer: (C) The atomic number equals the number of protons in the atomic nucleus.
Explanation: Atomic number counts nuclear protons and determines the chemical element. Rutherford’s scattering experiment revealed the nucleus, whereas Thomson discovered the electron and proposed a diffuse positive background. Mass number counts protons plus neutrons, not electrons.
3. Which transition of an electron in a hydrogen atom emits radiation of the shortest wavelength?
[UPSC NDA & NA (I) — 2012]
(A) n = 2 to n = 1
(B) n = 3 to n = 2
(C) n = 4 to n = 3
(D) n = 5 to n = 4
Answer: (A) n = 2 to n = 1
Explanation: Shorter wavelength means greater photon energy, because E = hc/λ. The n = 2 to n = 1 transition produces the largest energy drop among those listed for hydrogen. Therefore it emits the shortest-wavelength radiation.
2. Regarding an atom of a chemical element, what does the magnetic quantum number refer to?
[UPSC Civil Services Prelims — 2003]
(A) Orientation
(B) Shape
(C) Size
(D) Spin
Answer: (A) Orientation
Explanation: The magnetic quantum number, mₗ, distinguishes orientations of orbitals within a particular subshell. Its permitted values run from −l to +l. The azimuthal quantum number l controls orbital shape, while the spin quantum number describes electron spin.
1. Which of the following carries a negative charge?
[45th BPSC Preliminary Examination — 2002]
(A) X-rays
(B) Alpha particles
(C) Beta particles
(D) Gamma rays
Answer: (C) Beta particles
Explanation: In the conventional beta-minus form of nuclear radiation, a beta particle is a high-speed negatively charged electron. Alpha particles are positively charged helium nuclei; X-rays and gamma rays are electrically neutral photons. Beta-plus particles are positrons, so the qualification matters.