Atomic Structure and Chemical Bonding — GK Questions with Answers

1. Who discovered the electron?
   1. Rutherford
   2. J. J. Thomson
   3. Chadwick
   4. Millikan
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   Answer: (B) J. J. Thomson

   In 1897 Thomson bent cathode rays in electric and magnetic fields and proved they are negatively charged particles (electrons); Millikan later measured the charge.

2. By which reaction was the neutron discovered?
   1. α-particles on ¹⁴N
   2. Discharge in H₂
   3. Neutron on ²³⁵U
   4. α-particles on ⁹Be
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   Answer: (D) α-particles on ⁹Be

   Chadwick (1932): ⁹₄Be + ⁴₂He → ¹²₆C + ¹₀n.

3. Which statement about cathode rays is wrong?
   1. Their e/m changes with the gas
   2. They travel in straight lines
   3. They are made of negatively charged particles
   4. They bend towards the positive plate
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   Answer: (A) Their e/m changes with the gas

   The e/m of cathode rays is always the same; it is the e/m of anode (canal) rays that changes with the gas.

4. Number of electrons in ⁵⁶₂₆Fe³⁺?
   1. 26
   2. 29
   3. 23
   4. 30
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   Answer: (C) 23

   Neutral Fe has 26 electrons; 3 fewer for a +3 charge → 23.

5. What are ¹⁴C and ¹⁴N to each other?
   1. Isotopes
   2. Isotones
   3. Isobars
   4. Isoelectronic
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   Answer: (C) Isobars

   Both have mass number 14, but different atomic numbers (6 and 7) — isobars.

6. Which group is isoelectronic?
   1. F⁻, Ne, Mg²⁺
   2. Na⁺, K⁺, Rb⁺
   3. O, S, Se
   4. Cl⁻, Br⁻, I⁻
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   Answer: (A) F⁻, Ne, Mg²⁺

   F⁻, Ne and Mg²⁺ — all three have 10 electrons.

7. In Rutherford’s experiment most α-particles passed straight through. What did this show?
   1. The nucleus is positive
   2. The atom is indivisible
   3. Electrons are in the nucleus
   4. Most of the atom is empty
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   Answer: (D) Most of the atom is empty

   Passing straight shows empty space; bouncing back shows a small heavy positive nucleus.

8. What could Bohr’s model not explain?
   1. The line spectrum of hydrogen
   2. The Zeeman effect and spectra of many-electron atoms
   3. The stability of hydrogen
   4. The spectrum of He⁺
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   Answer: (B) The Zeeman effect and spectra of many-electron atoms

   Bohr’s model is limited to one-electron systems; it could not explain splitting of lines in a magnetic field (Zeeman) or many-electron spectra.

9. Energy of the second orbit (n = 2) of hydrogen?
   1. −3.4 eV
   2. −13.6 eV
   3. −1.51 eV
   4. −6.8 eV
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   Answer: (A) −3.4 eV

   Eₙ = −13.6/n² = −13.6/4 = −3.4 eV.

10. Which series of the hydrogen spectrum lies in the visible region?
    1. Lyman
    2. Paschen
    3. Balmer
    4. Pfund
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    Answer: (C) Balmer

    The Balmer series (n₁ = 2) is visible; Lyman is ultraviolet, the rest infrared.

11. Heisenberg’s uncertainty principle goes against the basic idea of which model?
    1. Quantum mechanical
    2. Bohr’s fixed orbits
    3. Molecular orbital
    4. VSEPR
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    Answer: (B) Bohr’s fixed orbits

    A fixed orbit means both position and momentum are fixed — not possible by the uncertainty principle.

12. Possible values of l for n = 3?
    1. 0, 1, 2, 3
    2. 0, 1
    3. 1, 2, 3
    4. 0, 1, 2
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    Answer: (D) 0, 1, 2

    l runs from 0 to n − 1; for n = 3, 0, 1, 2 (3s, 3p, 3d).

13. How many orbitals are in a d subshell?
    1. 3
    2. 1
    3. 7
    4. 5
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    Answer: (D) 5

    For l = 2, m has 2l + 1 = 5 values — five d orbitals, up to 10 electrons.

14. By which rule can an orbital hold no more than two electrons?
    1. Aufbau
    2. Pauli exclusion
    3. Hund
    4. (n + l) rule
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    Answer: (B) Pauli exclusion

    Pauli: no two electrons can have all four quantum numbers the same — so an orbital holds at most two, with opposite spins.

15. How many unpaired electrons does nitrogen (2p³) have?
    1. 1
    2. 2
    3. 3
    4. 0
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    Answer: (C) 3

    By Hund’s rule each of the three 2p orbitals gets one electron — three unpaired.

16. Correct electronic configuration of copper (Z = 29)?
    1. [Ar] 3d¹⁰ 4s¹
    2. [Ar] 3d⁹ 4s²
    3. [Ar] 3d¹⁰ 4s²
    4. [Ar] 4s² 4p⁹
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    Answer: (A) [Ar] 3d¹⁰ 4s¹

    A completely filled 3d¹⁰ is more stable, so one electron moves from 4s to 3d.

17. Which orbital fills before 3d?
    1. 4p
    2. 4s
    3. 5s
    4. 4d
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    Answer: (B) 4s

    (n + l): 4s = 4, 3d = 5 — 4s, with the lower (n + l), fills first.

18. Correct statement about gamma rays?
    1. They carry +2 charge
    2. They bend towards the negative plate in an electric field
    3. Their mass is 4 u
    4. Their penetrating power is the highest
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    Answer: (D) Their penetrating power is the highest

    Gamma rays are electromagnetic waves with no charge and no mass; highest penetration, least ionisation.

19. When an element emits one α-particle, the new element in the periodic table is —
    1. two places to the left
    2. one place to the right
    3. two places to the right
    4. in the same place
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    Answer: (A) two places to the left

    In α-decay Z falls by two, so the new element is two places to the left.

20. The element formed by emitting one β-particle is what of the original?
    1. Isotope
    2. Isotone
    3. Isobar
    4. The same element
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    Answer: (C) Isobar

    In β-decay A does not change and Z rises by one — equal mass number, so an isobar.

21. How many α and β particles are emitted when ²³⁸U becomes ²⁰⁶Pb?
    1. 6α, 8β
    2. 8α, 8β
    3. 8α, 6β
    4. 4α, 6β
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    Answer: (C) 8α, 6β

    α = (238 − 206)/4 = 8; β = 2 × 8 − (92 − 82) = 6.

22. A substance has a half-life of 5 days. How much of 16 g is left after 20 days?
    1. 1 g
    2. 2 g
    3. 4 g
    4. 8 g
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    Answer: (A) 1 g

    20/5 = 4 half-lives; 16 × (½)⁴ = 1 g.

23. Who developed carbon dating?
    1. Marie Curie
    2. Becquerel
    3. Rutherford
    4. Willard Libby
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    Answer: (D) Willard Libby

    Willard Libby (1949) — based on the 5730-year half-life of ¹⁴C; Chemistry Nobel 1960.

24. Which isotope is used to test and treat thyroid diseases?
    1. Cobalt-60
    2. Iodine-131
    3. Carbon-14
    4. Sodium-24
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    Answer: (B) Iodine-131

    The thyroid gland absorbs iodine, so radioactive I-131 collects there and serves both testing and treatment.

25. By which process is energy made in the Sun?
    1. Nuclear fusion
    2. Nuclear fission
    3. Radioactive decay
    4. Chemical burning
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    Answer: (A) Nuclear fusion

    In the Sun hydrogen nuclei join to form helium (4¹H → ⁴He) — nuclear fusion.

26. Why does MgO melt far higher than NaCl?
    1. MgO is covalent
    2. MgO is lighter
    3. Double charges on Mg²⁺ and O²⁻ make the lattice energy very high
    4. NaCl has hydrogen bonds
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    Answer: (C) Double charges on Mg²⁺ and O²⁻ make the lattice energy very high

    Higher charges and smaller ions → strong electrostatic attraction → high lattice energy and high melting point.

27. Number of σ and π bonds in the N₂ molecule?
    1. 3σ
    2. 1σ and 2π
    3. 2σ and 1π
    4. 3π
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    Answer: (B) 1σ and 2π

    A triple bond = one σ (head-on overlap) + two π (sideways).

28. Which of these has a coordinate bond?
    1. CH₄
    2. NaCl
    3. H₂O
    4. NH₄⁺
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    Answer: (D) NH₄⁺

    The nitrogen of NH₃ gives its lone pair to H⁺ to form NH₄⁺ — a donor–acceptor bond.

29. Which molecule has zero dipole moment?
    1. H₂O
    2. NH₃
    3. HCl
    4. CO₂
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    Answer: (D) CO₂

    CO₂ is linear and symmetric — the two C=O bond moments are equal and opposite, so they cancel.

30. Why does water boil far higher than H₂S?
    1. Water has a higher molecular mass
    2. Hydrogen bonds between water molecules
    3. H₂S is ionic
    4. Water has a π bond
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    Answer: (B) Hydrogen bonds between water molecules

    O is very electronegative, so water molecules stay joined by hydrogen bonds; this does not happen with S.

31. Cause of the lustre and electrical conductivity of metals?
    1. Hydrogen bonds
    2. Ionic bonds
    3. Free (delocalised) electrons
    4. van der Waals forces
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    Answer: (C) Free (delocalised) electrons

    Electron-sea model — free electrons carry current and reflect light.

32. What is the shape of the NH₃ molecule?
    1. Trigonal pyramidal
    2. Trigonal planar
    3. Tetrahedral
    4. Bent
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    Answer: (A) Trigonal pyramidal

    Three bond pairs and one lone pair — all four regions point tetrahedrally, but the atoms form a pyramid; angle 107°.

33. Shape of the XeF₄ molecule?
    1. Tetrahedral
    2. Square planar
    3. See-saw
    4. Octahedral
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    Answer: (B) Square planar

    Six electron regions (4 bond + 2 lone, sp³d²); the two lone pairs are above and below, so the four F lie in a square.

34. Hybridisation of carbon in ethene (C₂H₄)?
    1. sp
    2. sp³d
    3. sp³
    4. sp²
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    Answer: (D) sp²

    Each carbon forms three σ bonds (two H, one C) and one π bond — three regions, so sp² (120°).

35. Which of these molecules is paramagnetic?
    1. O₂
    2. N₂
    3. F₂
    4. H₂
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    Answer: (A) O₂

    In MO theory two electrons of O₂ stay unpaired in π* orbitals — that is why liquid oxygen sticks to a magnet.

36. Why does the He₂ molecule not form?
    1. Helium is very light
    2. Helium is radioactive
    3. Its bond order is zero
    4. Helium has no p orbital
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    Answer: (C) Its bond order is zero

    2 electrons in bonding σ1s and 2 in antibonding σ*1s — bond order ½(2 − 2) = 0.

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