50 Objective Questions on the History and Structure of the Atom

These 50 multiple-choice questions are based on the fundamental concepts of atomic theory from Dalton’s atomic theory and its modifications, through the discovery of subatomic particles, to modern understanding of atomic structure, electronic configuration, atomic number, mass number, molecules, ions, and atomicity.

Use these questions for revision, classroom quizzes, or exam preparation in Chemistry (especially SSCE, NECO, WAEC, and similar examinations).

Questions

1. The Greek word from which “atom” originates means
   A. Smallest particle B. Indivisible C. Invisible D. Fundamental
2. Who proposed the plum pudding model of the atom?
   A. Ernest Rutherford B. J.J. Thomson C. Niels Bohr D. John Dalton
3. In the plum pudding model, electrons were described as being embedded in
   A. A positive sphere B. A negative sphere C. A neutral sphere D. Empty space
4. Which scientist discovered the electron in 1897?
   A. Rutherford B. Chadwick C. J.J. Thomson D. Goldstein
5. Who discovered the proton?
   A. J.J. Thomson B. Eugene Goldstein C. James Chadwick D. Niels Bohr
6. The nucleus of the atom was discovered by
   A. J.J. Thomson B. Niels Bohr C. Ernest Rutherford D. John Dalton
7. Who discovered the neutron?
   A. Ernest Rutherford B. James Chadwick C. J.J. Thomson D. Niels Bohr
8. According to John Dalton (1805/1808), atoms of the same element are
   A. Different in mass B. Alike in every aspect C. Different in size D. Always combined
9. A molecule is the smallest particle of a substance that can
   A. Take part in a chemical reaction B. Exist independently and retain the properties of that substance C. Carry an electric charge D. Be split into protons
10. The atomicity of neon (Ne) is
    A. 1 B. 2 C. 4 D. 8
11. Which element has atomicity of 8?
    A. Phosphorus B. Sulphur C. Oxygen D. Nitrogen
12. Which of the following is a polyatomic molecule?
    A. O₂ B. N₂ C. S₈ D. HCl
13. Which element is diatomic?
    A. Neon B. Argon C. Oxygen D. Helium
14. Positively charged ions are called
    A. Anions B. Cations C. Molecules D. Neutrons
15. Which of the following is an anion?
    A. Na⁺ B. Ca²⁺ C. Cl⁻ D. NH₄⁺
16. Which of Dalton’s postulates states that atoms can neither be created nor destroyed?
    A. First B. Second C. Third D. Fourth
17. Which modification of Dalton’s theory was caused by the discovery of isotopes?
    A. Atoms are indivisible B. Atoms of the same element are identical C. Atoms combine in simple ratios D. Atoms cannot be destroyed
18. The discovery of which particle showed that atoms are divisible?
    A. Electron B. Proton C. Neutron D. All three
19. In nuclear reactions (e.g. fission of uranium-235), atoms can be
    A. Created B. Split into simpler atoms C. Completely destroyed D. Turned into energy only
20. The three fundamental particles of an atom are
    A. Proton, electron, positron B. Proton, neutron, electron C. Nucleus, electron, neutron D. Proton, nucleon, electron
21. The proton and neutron are located in the
    A. Electron cloud B. Nucleus C. Outer shell D. K shell
22. The charge on an electron is
    A. Positive B. Neutral C. Negative D. Variable
23. The mass of a neutron is approximately equal to the mass of
    A. Electron B. Proton C. Hydrogen atom D. Both B and C
24. According to Niels Bohr, electrons move in
    A. Random paths B. Circular orbits/shells C. Straight lines D. Elliptical paths only
25. The letter used to represent the first electron shell is
    A. L B. M C. K D. N
26. The maximum number of electrons in the K shell is
    A. 2 B. 8 C. 18 D. 32
27. The maximum number of electrons in the L shell is
    A. 2 B. 8 C. 18 D. 32
28. The formula for the maximum number of electrons in a shell is
    A. 2n B. 2n² C. n² D. 8n
29. The electronic configuration of sodium (atomic number 11) is
    A. 2,8,1 B. 2,8,2 C. 2,8,3 D. 2,8,8,1
30. Which element has the electronic configuration 2,8,8,2?
    A. Argon B. Potassium C. Calcium D. Chlorine
31. The element with electronic configuration 2,8,6 is
    A. Nitrogen B. Oxygen C. Fluorine D. Sulphur
32. Which atom has the highest number of electrons in its outermost shell?
    A. Oxygen (₈O) B. Neon (₁₀Ne) C. Phosphorus (₁₅P) D. Potassium (₁₉K)
33. The atomic number of an element is equal to the number of
    A. Neutrons B. Protons C. Electrons + neutrons D. Nucleons
34. The mass number is the sum of
    A. Protons and electrons B. Protons and neutrons C. Neutrons and electrons D. Protons only
35. In the symbol ₂₀Y⁴⁰, the atomic number is
    A. 20 B. 40 C. 60 D. 0
36. In the symbol ₂₁X⁴⁵, the number of neutrons is
    A. 21 B. 24 C. 45 D. 66
37. An atom of fluorine is written as ₉F¹⁹. How many neutrons does it have?
    A. 9 B. 10 C. 19 D. 28
38. The atomic number of an element whose cation Y⁺ has configuration 1s² 2s² 2p⁶ is
    A. 9 B. 10 C. 11 D. 12
39. Relative atomic mass is not usually a whole number because of the existence of
    A. Isobars B. Isotopes C. Ions D. Molecules
40. In a neutral atom, the number of protons is equal to the number of
    A. Neutrons B. Electrons C. Nucleons D. Ions
41. Which of the following is not true about Dalton’s atomic theory?
    A. Atoms are indivisible B. Atoms of different elements are different C. Atoms combine in simple whole number ratios D. Atoms can be split in chemical reactions
42. Which molecule shows triatomic structure?
    A. O₂ B. O₃ C. P₄ D. S₈
43. The ammonium ion (NH₄⁺) is an example of
    A. Monoatomic cation B. Polyatomic cation C. Monoatomic anion D. Polyatomic anion
44. The electronic configuration of argon is
    A. 2,8,1 B. 2,8,7 C. 2,8,8 D. 2,8,8,1
45. Which shell has the lowest energy level?
    A. L B. M C. N D. K
46. The maximum number of electrons in the N shell is
    A. 8 B. 18 C. 32 D. 2
47. An element with atomic number 17 has how many electrons in its outermost shell?
    A. 1 B. 5 C. 7 D. 8
48. The relative atomic mass is compared to the mass of
    A. Hydrogen-1 B. Carbon-12 C. Oxygen-16 D. Nitrogen-14
49. In the symbol ₁₂Mg²⁴, the number of electrons in the neutral atom is
    A. 12 B. 24 C. 36 D. 10
50. Which of the following statements about molecules is correct?
    A. Molecules are always made of ionic bonds B. Molecules of elements are always diatomic C. Molecules are held together by covalent bonds D. Molecules cannot exist independently

Answers with Short Explanations

1. B – The word “atom” comes from Greek “atomos” meaning “indivisible” or “uncuttable”.
2. B – J.J. Thomson proposed the plum pudding model in 1897.
3. A – Electrons were thought to be embedded in a positively charged sphere like plums in pudding.
4. C – J.J. Thomson discovered the electron through cathode ray experiments.
5. B – Eugene Goldstein discovered positively charged particles (protons) in canal rays.
6. C – Ernest Rutherford discovered the nucleus through his gold foil experiment.
7. B – James Chadwick discovered the neutron in 1932.
8. B – Dalton stated that atoms of the same element are identical in all respects.
9. B – This is the standard definition of a molecule.
10. A – Neon is monoatomic (Ne) – atomicity = 1.
11. B – Sulphur exists as S₈ → atomicity = 8.
12. C – S₈ has 8 atoms → polyatomic.
13. C – Oxygen exists as O₂ → diatomic.
14. B – Cations are positively charged ions.
15. C – Cl⁻ is negatively charged → anion.
16. B – This is the second postulate of Dalton’s theory.
17. B – Isotopes have different masses but same chemical properties → disproved identical atoms.
18. D – Discovery of proton, neutron, and electron proved atoms are divisible.
19. B – Nuclear fission splits heavy atoms into lighter ones.
20. B – The three fundamental particles are proton, neutron, and electron.
21. B – Protons and neutrons form the nucleus (Rutherford’s model).
22. C – Electron carries a negative charge.
23. B – Neutron and proton have almost equal mass.
24. B – Bohr proposed electrons move in fixed circular orbits.
25. C – The innermost shell is called K shell.
26. A – K shell holds maximum 2 electrons (2n² where n=1).
27. B – L shell (n=2) holds maximum 8 electrons.
28. B – The formula is 2n².
29. A – Sodium (11 electrons): 2,8,1
30. C – Calcium (20 electrons): 2,8,8,2
31. D – Sulphur (16 electrons): 2,8,6
32. B – Neon has a full outermost shell with 8 electrons (stable).
33. B – Atomic number = number of protons.
34. B – Mass number = protons + neutrons.
35. A – The lower number (20) is the atomic number.
36. B – Neutrons = mass number – atomic number = 45 – 21 = 24
37. B – Neutrons = 19 – 9 = 10
38. C – Y⁺ has 10 electrons → neutral atom has 11 electrons → atomic number = 11
39. B – Isotopes cause relative atomic mass to be non-integer.
40. B – In neutral atoms, protons = electrons.
41. D – Atoms are not split in ordinary chemical reactions (only in nuclear reactions).
42. B – Ozone (O₃) has three atoms → triatomic.
43. B – NH₄⁺ is a group of atoms with positive charge → polyatomic cation.
44. C – Argon (18 electrons): 2,8,8
45. D – K shell is closest to nucleus → lowest energy.
46. C – N shell (n=4): 2(4)² = 32 electrons maximum.
47. C – Chlorine (atomic number 17): 2,8,7 → 7 electrons in outer shell.
48. B – Carbon-12 is the standard reference for relative atomic mass.
49. A – Neutral Mg has 12 protons = 12 electrons.
50. C – Molecules (especially of elements & compounds) are held by covalent bonds.