100 Short Q&A for Atomic Structure (Class 11 NCERT)

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Atomic Structure

100 Short Q&A • NCERT Class 11 Chemistry

Class 11 JEE / NEET

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100 Short Q&A for Atomic Structure

Part 1: Fundamental Particles and Early Models
1. Q1: Who discovered the electron?
   A: J. J. Thomson (using Cathode Ray Tube).
2. Q2: Who discovered the proton?
   A: E. Goldstein (using Canal Rays).
3. Q3: Who discovered the neutron?
   A: James Chadwick.
4. Q4: What is the charge-to-mass ratio (e/m) of an electron?
   A: 1.758820 × 10¹¹ C/kg.
5. Q5: What is the charge of an electron?
   A: −1.602 × 10⁻¹⁹ Coulombs (Millikan's Oil Drop method).
6. Q6: What is the mass of a proton (in kg)?
   A: Approximately 1.672 × 10⁻²⁷ kg.
7. Q7: What is an α-particle?
   A: A doubly positively charged helium nucleus (⁴He²⁺).
8. Q8: What did Thomson's model of the atom propose?
   A: A "Plum Pudding" or "Watermelon" model with embedded electrons.
9. Q9: What was the major observation of Rutherford's α-scattering experiment?
   A: Most α-particles passed straight through the gold foil.
10. Q10: What is the dense, central part of an atom called?
    A: The Nucleus.
11. Q11: What did Rutherford's model fail to explain?
    A: The stability of the atom.
12. Q12: Define Atomic Number (Z).
    A: The number of protons in the nucleus.
13. Q13: Define Mass Number (A).
    A: The total number of protons and neutrons (nucleons).
14. Q14: What are Isotopes?
    A: Atoms of the same element with the same Z but different A (different neutrons).
15. Q15: What are Isobars?
    A: Atoms with the same A but different Z (different elements).
16. Q16: What are Isotones?
    A: Atoms with the same number of neutrons.
17. Q17: What are isoelectronic species?
    A: Species (atoms/ions) having the same number of electrons.
18. Q18: Write the symbol for an element X with Z=17 and A=35.
    A: ³⁵₁₇Cl (Chlorine).
19. Q19: How many neutrons are in a ²³⁵U atom?
    A: 235 − 92 = 143 neutrons.
20. Q20: Who proposed that energy is quantized?
    A: Max Planck.
Part 2: Dual Nature of Matter and Electromagnetic Radiation
21. Q21: What is the velocity of electromagnetic radiation in a vacuum?
    A: The speed of light (c), approximately 3.0 × 10⁸ m/s.
22. Q22: What is the relationship between frequency (ν) and wavelength (λ)?
    A: c = νλ.
23. Q23: Which region of the electromagnetic spectrum has the highest frequency?
    A: Gamma Rays.
24. Q24: What is the SI unit of frequency?
    A: Hertz (Hz) or s⁻¹.
25. Q25: What is the energy of one photon given by (Planck's Equation)?
    A: E = hν (where h is Planck's constant).
26. Q26: What is the value of Planck's constant (h)?
    A: 6.626 × 10⁻³⁴ J s.
27. Q27: What is a Black Body?
    A: An ideal body that absorbs and emits all frequencies of radiation equally.
28. Q28: What is the Photoelectric Effect?
    A: The ejection of electrons when light of sufficient frequency strikes a metal surface.
29. Q29: What is Threshold Frequency (ν₀)?
    A: The minimum frequency of light required to cause the photoelectric effect.
30. Q30: What are the two aspects of the Dual Nature of Electromagnetic Radiation?
    A: Wave nature and Particle nature.
31. Q31: What is a spectrum?
    A: The display of components of an electromagnetic radiation arranged according to frequency or wavelength.
32. Q32: What is an emission spectrum?
    A: The spectrum of radiation emitted by a substance that has absorbed energy.
33. Q33: What is a line spectrum?
    A: A spectrum consisting of sharp, well-defined lines (characteristic of atoms).
34. Q34: What type of spectrum is the hydrogen spectrum?
    A: A line emission spectrum.
35. Q35: Which series in the hydrogen spectrum lies in the UV region?
    A: The Lyman series.
36. Q36: Which series in the hydrogen spectrum lies in the visible region?
    A: The Balmer series.
37. Q37: State the de Broglie hypothesis.
    A: Matter, like radiation, exhibits dual behavior (wave and particle).
38. Q38: Write the de Broglie wavelength equation.
    A: λ = h/(mv) (where p=mv is momentum).
39. Q39: Which particle shows wave properties even at ordinary speeds?
    A: Microscopic particles like electrons.
40. Q40: What is the energy of the electron in the n=1 state of a hydrogen atom?
    A: −2.18 × 10⁻¹⁸ J or −13.6 eV.
Part 3: Bohr's Model and Quantum Mechanical Model
41. Q41: What is the main postulate of Bohr's model regarding electron orbits?
    A: Electrons revolve only in certain fixed, non-radiating orbits (stationary states).
42. Q42: What is the condition for the angular momentum of an electron in Bohr's model?
    A: It must be an integral multiple of h/(2π), i.e., mvr = n⋅h/(2π).
43. Q43: What happens when an electron jumps from a lower to a higher orbit?
    A: Energy is absorbed.
44. Q44: What happens when an electron drops from a higher to a lower orbit?
    A: Energy is emitted (as a photon).
45. Q45: Give the Rydberg formula for the wavelength of hydrogen spectral lines.
    A: 1/λ = R_H(1/n₁² − 1/n₂²).
46. Q46: What is the value of the Rydberg constant (R_H)?
    A: 109677 cm⁻¹.
47. Q47: What is the main limitation of Bohr's model?
    A: It could not explain the spectra of multi-electron atoms.
48. Q48: What is the Heisenberg Uncertainty Principle?
    A: It is impossible to determine simultaneously and exactly the position and momentum of a microscopic particle.
49. Q49: Write the mathematical expression for the Uncertainty Principle.
    A: Δx ⋅ Δp ≥ h/(4π).
50. Q50: What is the region of space around the nucleus where the probability of finding an electron is maximum?
    A: An Orbital.
51. Q51: What are the three Quantum Numbers derived from the Schrödinger Wave Equation?
    A: Principal (n), Azimuthal (l), and Magnetic (m_l).
52. Q52: What does the Principal Quantum Number (n) determine?
    A: The size and energy of the orbit/shell.
53. Q53: What are the possible values of n?
    A: 1, 2, 3, 4, … (or K, L, M, N, …).
54. Q54: What does the Azimuthal (or Angular Momentum) Quantum Number (l) determine?
    A: The shape of the orbital (subshell).
55. Q55: What are the possible values of l for a given n?
    A: 0 to (n−1).
56. Q56: What are the subshells corresponding to l=0, 1, 2, 3?
    A: s, p, d, f.
57. Q57: What does the Magnetic Quantum Number (m_l) determine?
    A: The orientation of the orbital in space.
58. Q58: What are the possible values of m_l for a given l?
    A: −l to +l (including 0).
59. Q59: How many orbitals are in the d-subshell (l=2)?
    A: Five (m_l = −2, −1, 0, +1, +2).
60. Q60: What does the Spin Quantum Number (m_s) determine?
    A: The orientation of the electron's spin.
61. Q61: What are the two possible values of m_s?
    A: +1/2 (spin up) and −1/2 (spin down).
62. Q62: What is the shape of an s orbital?
    A: Spherical.
63. Q63: What is the shape of a p orbital?
    A: Dumb-bell shaped.
64. Q64: How many nodal planes does a p orbital have?
    A: One (a plane passing through the nucleus).
65. Q65: Define a Radial Node.
    A: A spherical surface where the probability of finding the electron is zero.
66. Q66: What is the formula for the total number of nodes?
    A: n − 1.
67. Q67: What is the formula for the number of angular nodes?
    A: l.
68. Q68: What is the formula for the number of radial nodes?
    A: n − l − 1.
69. Q69: What is the total number of nodes for a 3p orbital?
    A: 3 − 1 = 2 (1 radial, 1 angular).
70. Q70: How many degenerate orbitals are present in the M-shell (n=3)?
    A: 3² = 9 (one 3s, three 3p, five 3d).
Part 4: Electronic Configuration Rules
71. Q71: State the Aufbau Principle.
    A: Electrons are added one by one to orbitals with the lowest energy first.
72. Q72: What is the (n+l) rule used for?
    A: To determine the relative energy of orbitals.
73. Q73: Which orbital has lower energy: 4s or 3d?
    A: 4s (n+l is 4+0=4, vs 3+2=5).
74. Q74: State Pauli's Exclusion Principle.
    A: No two electrons in an atom can have the same set of all four quantum numbers.
75. Q75: What is the maximum number of electrons in an orbital?
    A: Two (with opposite spins).
76. Q76: State Hund's Rule of Maximum Multiplicity.
    A: Pairing of electrons in a degenerate set of orbitals (p, d, f) does not occur until each orbital has one electron (singly occupied).
77. Q77: Write the electronic configuration of Nitrogen (Z=7).
    A: 1s² 2s² 2p³.
78. Q78: What is the total number of unpaired electrons in Oxygen (Z=8)?
    A: Two (↑↓ in 2s, ↑↓↑↑ in 2p).
79. Q79: Why is the electronic configuration of Chromium (Z=24) exceptional?
    A: Half-filled orbitals are more stable, so it is [Ar] 3d⁵ 4s¹ instead of 3d⁴ 4s².
80. Q80: Why is the electronic configuration of Copper (Z=29) exceptional?
    A: Fully-filled orbitals are more stable, so it is [Ar] 3d¹⁰ 4s¹ instead of 3d⁹ 4s².
81. Q81: Write the electronic configuration of Na⁺ ion (Z=11).
    A: 1s² 2s² 2p⁶ (Neon configuration).
82. Q82: How many orbitals are singly occupied in the ground state of Chlorine (Z=17)?
    A: One (3p orbital).
83. Q83: What is the order of filling of electrons in the 5th period according to the (n+l) rule?
    A: 5s, 4d, 5p (then 6s, 4f, …).
84. Q84: What is the general electronic configuration for the p-block elements?
    A: ns² np¹⁻⁶.
85. Q85: What is the highest value of n for an electron in the ground state of Potassium (Z=19)?
    A: n=4 (4s¹).
86. Q86: What is the maximum number of electrons in the n=4 shell?
    A: 2n² = 2(4²) = 32 electrons.
87. Q87: How is the energy of orbitals determined in multi-electron atoms?
    A: By the (n+l) rule (and n if (n+l) is equal).
88. Q88: What is the minimum energy required to remove an electron from the ground state of an atom?
    A: Ionization Enthalpy (or Ionization Energy).
89. Q89: What is the magnetic behavior of a species with unpaired electrons?
    A: Paramagnetic.
90. Q90: What is the magnetic behavior of a species with all paired electrons?
    A: Diamagnetic.
91. Q91: Is N₂ (Nitrogen molecule) paramagnetic or diamagnetic?
    A: Diamagnetic (all electrons are paired).
92. Q92: What is a Wave Function (ψ)?
    A: A mathematical function that contains all the information about an electron in an atom.
93. Q93: What does ψ² represent?
    A: The probability density of finding the electron at a point.
94. Q94: Which quantum number is not obtained from the Schrödinger equation?
    A: The Spin Quantum Number (m_s).
95. Q95: What is the relationship between the radius of the n-th orbit and n in Bohr's model?
    A: r_n ∝ n² (for H-like species).
96. Q96: Define Atomic Mass Unit (amu) or u.
    A: 1/12 of the mass of a C-12 atom.
97. Q97: What is the term for the stability gained by half-filled and fully-filled subshells?
    A: Exchange Energy (or greater symmetry).
98. Q98: In a hydrogen atom, what determines the energy of an electron?
    A: Only the Principal Quantum Number (n).
99. Q99: What is the frequency of light when an electron jumps from n=3 to n=2 in a H-atom?
    A: The first line of the Balmer series (Red light).
100. Q100: What are the main characteristics of a cathode ray?
     A: They are streams of electrons; travel in straight lines; and are deflected by electric and magnetic fields.

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