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50 MCQs on Structure of the Atom with Answers & Explanations

Explore 50 MCQs on the Structure of the Atom with answers and explanations. Perfect for students preparing for exams. Visit The Govt Guide for more!
50 MCQs on Structure of the Atom with Answers & Explanations

Master the concepts of atomic structure with these 50 multiple-choice questions (MCQs) designed for students preparing for exams. Covering topics like subatomic particles, atomic models, valency, isotopes, and isobars, this blog post includes detailed explanations to enhance your understanding. Authored by Rajesh Jaipal for The Govt Guide.

Structure of the Atom MCQs

MCQs on Structure of the Atom

1. Who discovered the electron?
a) E. Goldstein
b) J.J. Thomson
c) Ernest Rutherford
d) J. Chadwick
Correct Answer: b) J.J. Thomson
J.J. Thomson discovered the electron in 1897 through his cathode ray tube experiments, identifying it as a negatively charged subatomic particle.
2. What are canal rays?
a) Negatively charged particles
b) Positively charged radiations
c) Neutral particles
d) Electromagnetic waves
Correct Answer: b) Positively charged radiations
Canal rays, discovered by E. Goldstein in 1886, are positively charged radiations observed in gas discharge tubes, leading to the discovery of the proton.
3. If an atom contains one electron and one proton, what is its charge?
a) Positive
b) Negative
c) Neutral
d) Variable
Correct Answer: c) Neutral
An atom with one electron (negative charge) and one proton (positive charge) is electrically neutral because the charges balance each other.
4. What is the mass of a proton compared to an electron?
a) Equal
b) 2000 times greater
c) 1000 times greater
d) Negligible
Correct Answer: b) 2000 times greater
The mass of a proton is approximately 2000 times that of an electron, as stated in the text.
5. According to Thomson’s model, how is an atom structured?
a) Electrons revolve around a nucleus
b) Electrons embedded in a positive sphere
c) Protons scattered in a negative cloud
d) Neutrons in the center
Correct Answer: b) Electrons embedded in a positive sphere
Thomson’s model compares an atom to a Christmas pudding, with electrons embedded in a sphere of positive charge, making the atom neutral.
6. Why did Rutherford choose a gold foil for his experiment?
a) It is highly reactive
b) It can be made very thin
c) It is a poor conductor
d) It is magnetic
Correct Answer: b) It can be made very thin
Rutherford used a gold foil because it could be made extremely thin (about 1000 atoms thick), allowing alpha particles to pass through or be deflected.
7. What was the unexpected result of Rutherford’s alpha-particle scattering experiment?
a) All particles passed through
b) Some particles were deflected by 180°
c) No particles were deflected
d) All particles were absorbed
Correct Answer: b) Some particles were deflected by 180°
Rutherford observed that a very small fraction (1 in 12,000) of alpha particles rebounded, indicating a concentrated positive charge in the nucleus.
8. What does Rutherford’s model suggest about the atom’s structure?
a) Electrons are embedded in a positive sphere
b) A small, dense nucleus with electrons orbiting it
c) Protons are spread throughout the atom
d) Neutrons form the outer shell
Correct Answer: b) A small, dense nucleus with electrons orbiting it
Rutherford’s model proposed a small, positively charged nucleus containing most of the atom’s mass, with electrons revolving around it.
9. What is a major drawback of Rutherford’s model?
a) It could not explain atomic mass
b) It predicted that electrons would spiral into the nucleus
c) It ignored the presence of neutrons
d) It suggested atoms are indivisible
Correct Answer: b) It predicted that electrons would spiral into the nucleus
Rutherford’s model suggested that electrons orbiting the nucleus would lose energy and spiral inward, making the atom unstable, which contradicts the stability of matter.
10. What is a key feature of Bohr’s model of the atom?
a) Electrons radiate energy while orbiting
b) Electrons move in discrete orbits without radiating energy
c) The nucleus contains electrons
d) Protons orbit the nucleus
Correct Answer: b) Electrons move in discrete orbits without radiating energy
Bohr proposed that electrons move in specific orbits with fixed energy levels and do not radiate energy while in these orbits, addressing Rutherford’s model’s instability issue.
11. Who discovered the neutron?
a) J.J. Thomson
b) E. Goldstein
c) J. Chadwick
d) Neils Bohr
Correct Answer: c) J. Chadwick
J. Chadwick discovered the neutron in 1932, identifying it as a neutral particle with a mass similar to that of a proton.
12. How many electrons can the K-shell hold according to the $2n^2$ rule?
a) 2
b) 8
c) 18
d) 32
Correct Answer: a) 2
According to the $2n^2$ rule, the K-shell (n=1) can hold $2 \times 1^2 = 2$ electrons.
13. What is the maximum number of electrons in the L-shell?
a) 2
b) 8
c) 18
d) 32
Correct Answer: b) 8
The L-shell (n=2) can hold $2 \times 2^2 = 8$ electrons, as per the $2n^2$ rule.
14. What are valence electrons?
a) Electrons in the nucleus
b) Electrons in the innermost shell
c) Electrons in the outermost shell
d) Electrons shared between atoms
Correct Answer: c) Electrons in the outermost shell
Valence electrons are the electrons in the outermost shell of an atom, which determine its chemical reactivity and valency.
15. What is the valency of an element with a completely filled outermost shell?
a) 0
b) 1
c) 2
d) 8
Correct Answer: a) 0
Elements with a fully filled outermost shell (e.g., noble gases like helium and neon) have a valency of 0 because they are chemically inert.
16. What is the valency of magnesium?
a) 1
b) 2
c) 3
d) 4
Correct Answer: b) 2
Magnesium has 2 electrons in its outermost shell (Table 4.1), so its valency is 2, as it can lose these electrons to achieve a stable octet.
17. How is the valency of fluorine determined?
a) By losing 7 electrons
b) By gaining 1 electron
c) By sharing 7 electrons
d) By losing 1 electron
Correct Answer: b) By gaining 1 electron
Fluorine has 7 electrons in its outermost shell. It is easier to gain 1 electron to achieve an octet than to lose 7, so its valency is 1.
18. What is the atomic number of an element?
a) Number of neutrons
b) Number of protons
c) Number of electrons
d) Total number of nucleons
Correct Answer: b) Number of protons
The atomic number (Z) is the number of protons in the nucleus, which defines the element.
19. What is the mass number of an atom?
a) Number of electrons
b) Number of protons
c) Sum of protons and neutrons
d) Number of valence electrons
Correct Answer: c) Sum of protons and neutrons
The mass number (A) is the total number of protons and neutrons (nucleons) in the nucleus of an atom.
20. What is the atomic number of carbon?
a) 6
b) 12
c) 14
d) 8
Correct Answer: a) 6
Carbon has 6 protons in its nucleus, so its atomic number is 6 (Table 4.1).
21. How many neutrons does a helium atom have?
a) 1
b) 2
c) 3
d) 4
Correct Answer: b) 2
Helium has an atomic mass of 4 u and 2 protons. Thus, neutrons = 4 - 2 = 2 (Table 4.1).
22. Which particles are called nucleons?
a) Electrons and protons
b) Protons and neutrons
c) Electrons and neutrons
d) Electrons, protons, and neutrons
Correct Answer: b) Protons and neutrons
Protons and neutrons are located in the nucleus and are collectively called nucleons.
23. What are isotopes?
a) Atoms with the same mass number but different atomic numbers
b) Atoms with the same atomic number but different mass numbers
c) Atoms with different protons and electrons
d) Atoms with the same number of neutrons
Correct Answer: b) Atoms with the same atomic number but different mass numbers
Isotopes are atoms of the same element with the same number of protons (atomic number) but different numbers of neutrons, leading to different mass numbers.
24. Which of the following is an isotope of hydrogen?
a) Protium
b) Helium
c) Lithium
d) Carbon
Correct Answer: a) Protium
Hydrogen has three isotopes: protium (${}_1^1\text{H}$), deuterium (${}_1^2\text{H}$), and tritium (${}_1^3\text{H}$). Protium is one of them.
25. What are isobars?
a) Atoms with the same atomic number
b) Atoms with the same mass number but different atomic numbers
c) Atoms with the same number of electrons
d) Atoms with the same number of neutrons
Correct Answer: b) Atoms with the same mass number but different atomic numbers
Isobars are atoms of different elements with the same mass number but different atomic numbers, e.g., calcium (${}_{20}^{40}\text{Ca}$) and argon (${}_{18}^{40}\text{Ar}$).
26. What is the average atomic mass of chlorine?
a) 35 u
b) 37 u
c) 35.5 u
d) 36 u
Correct Answer: c) 35.5 u
Chlorine has two isotopes, ${}_{17}^{35}\text{Cl}$ (75%) and ${}_{17}^{37}\text{Cl}$ (25%). Average mass = $(35 \times 0.75) + (37 \times 0.25) = 35.5$ u.
27. Which isotope of uranium is used in nuclear reactors?
a) Uranium-235
b) Uranium-238
c) Uranium-234
d) Uranium-236
Correct Answer: a) Uranium-235
Uranium-235 is used as a fuel in nuclear reactors due to its ability to sustain a nuclear chain reaction.
28. What is the electronic configuration of sodium?
a) 2, 8, 1
b) 2, 8
c) 2, 1, 8
d) 8, 2, 1
Correct Answer: a) 2, 8, 1
Sodium (atomic number 11) has 11 electrons distributed as 2 in K-shell, 8 in L-shell, and 1 in M-shell (Table 4.1).
29. How many electrons does $\text{Na}^+$ have?
a) 11
b) 10
c) 12
d) 9
Correct Answer: b) 10
Sodium (Na) has 11 electrons. The $\text{Na}^+$ ion has lost 1 electron, leaving 10 electrons (2, 8 configuration).
30. Why is the $\text{Na}^+$ ion stable?
a) It has a positive charge
b) It has completely filled K and L shells
c) It has more neutrons
d) It has fewer protons
Correct Answer: b) It has completely filled K and L shells
The $\text{Na}^+$ ion has 10 electrons (2 in K-shell, 8 in L-shell), achieving a stable octet configuration.
31. What is the mass number of sulphur?
a) 16 u
b) 32 u
c) 17 u
d) 33 u
Correct Answer: b) 32 u
Sulphur has 16 protons and 16 neutrons, so its mass number is $16 + 16 = 32$ u (Table 4.1).
32. Which subatomic particle is not present in the hydrogen-1 (protium) nucleus?
a) Proton
b) Neutron
c) Electron
d) Nucleon
Correct Answer: b) Neutron
Protium (${}_1^1\text{H}$) has 1 proton and no neutrons in its nucleus.
33. What is the valency of carbon?
a) 2
b) 3
c) 4
d) 5
Correct Answer: c) 4
Carbon has 4 electrons in its outermost shell (Table 4.1), so its valency is 4, as it can share 4 electrons to achieve an octet.
34. Which model explains the stability of atoms?
a) Thomson’s model
b) Rutherford’s model
c) Bohr’s model
d) Dalton’s model
Correct Answer: c) Bohr’s model
Bohr’s model explains atomic stability by proposing that electrons occupy discrete orbits without radiating energy, unlike Rutherford’s model.
35. What is the charge of a neutron?
a) Positive
b) Negative
c) Neutral
d) Variable
Correct Answer: c) Neutral
Neutrons have no charge, as discovered by J. Chadwick.
36. How many electrons can the M-shell hold?
a) 2
b) 8
c) 18
d) 32
Correct Answer: c) 18
The M-shell (n=3) can hold $2 \times 3^2 = 18$ electrons, as per the $2n^2$ rule.
37. What is the electronic configuration of carbon?
a) 2, 4
b) 2, 8
c) 2, 6
d) 2, 2
Correct Answer: a) 2, 4
Carbon (atomic number 6) has 6 electrons: 2 in K-shell and 4 in L-shell (Table 4.1).
38. Which element has a valency of 0?
a) Sodium
b) Neon
c) Fluorine
d) Oxygen
Correct Answer: b) Neon
Neon has a fully filled outermost shell with 8 electrons (Table 4.1), making it chemically inert with a valency of 0.
39. What is the mass number of oxygen?
a) 8 u
b) 16 u
c) 18 u
d) 14 u
Correct Answer: b) 16 u
Oxygen has 8 protons and 8 neutrons, so its mass number is $8 + 8 = 16$ u (Table 4.1).
40. Which particle was discovered through the alpha-particle scattering experiment?
a) Electron
b) Proton
c) Neutron
d) Nucleus
Correct Answer: d) Nucleus
Rutherford’s alpha-particle scattering experiment led to the discovery of the atomic nucleus, a small, dense, positively charged center.
41. What is the valency of oxygen?
a) 1
b) 2
c) 3
d) 6
Correct Answer: b) 2
Oxygen has 6 electrons in its outermost shell and gains 2 electrons to achieve an octet, so its valency is 2 (Table 4.1).
42. What is the atomic number of an atom with 8 protons and 8 electrons?
a) 8
b) 16
c) 10
d) 6
Correct Answer: a) 8
The atomic number is the number of protons, which is 8 in this case.
43. Which element has an atomic mass of 27 u?
a) Carbon
b) Aluminium
c) Sulphur
d) Sodium
Correct Answer: b) Aluminium
Aluminium has 13 protons and 14 neutrons, giving a mass number of $13 + 14 = 27$ u (Table 4.1).
44. Which of the following pairs are isobars?
a) ${}_6^{12}\text{C}$ and ${}_6^{14}\text{C}$
b) ${}_{20}^{40}\text{Ca}$ and ${}_{18}^{40}\text{Ar}$
c) ${}_1^1\text{H}$ and ${}_1^2\text{H}$
d) ${}_{17}^{35}\text{Cl}$ and ${}_{17}^{37}\text{Cl}$
Correct Answer: b) ${}_{20}^{40}\text{Ca}$ and ${}_{18}^{40}\text{Ar}$
Isobars have the same mass number but different atomic numbers. Calcium (${}_{20}^{40}\text{Ca}$) and argon (${}_{18}^{40}\text{Ar}$) both have a mass number of 40.
45. What is the average atomic mass of bromine?
a) 79 u
b) 80 u
c) 81 u
d) 82 u
Correct Answer: b) 80 u
Bromine has two isotopes: ${}_{35}^{79}\text{Br}$ (49.7%) and ${}_{35}^{81}\text{Br}$ (50.3%). Average mass = $(79 \times 0.497) + (81 \times 0.503) \approx 80$ u.
46. If an element’s atomic number is 3, what is its valency?
a) 1
b) 2
c) 3
d) 0
Correct Answer: a) 1
The element with atomic number 3 is lithium, which has 1 electron in its outermost shell, giving it a valency of 1 (Table 4.1).
47. What is the name of the element with atomic number 3?
a) Helium
b) Lithium
c) Beryllium
d) Boron
Correct Answer: b) Lithium
The element with atomic number 3 is lithium (Table 4.1).
48. How many neutrons are in the nucleus of species X with 6 protons and 6 neutrons?
a) 6
b) 8
c) 12
d) 0
Correct Answer: a) 6
Species X has 6 neutrons, as given in the composition (6 protons + 6 neutrons).
49. What is the relation between species X (6 protons, 6 neutrons) and Y (6 protons, 8 neutrons)?
a) Isotopes
b) Isobars
c) Different elements
d) Identical atoms
Correct Answer: a) Isotopes
Species X and Y have the same atomic number (6 protons) but different mass numbers (12 and 14), making them isotopes of carbon.
50. Which statement is true about isotopes?
a) They have different chemical properties
b) They have the same number of neutrons
c) They have the same physical properties
d) They have the same atomic number
Correct Answer: d) They have the same atomic number
Isotopes of an element have the same atomic number (same number of protons) but different mass numbers due to varying numbers of neutrons.

Conclusion

These 50 MCQs cover the essential concepts of the structure of the atom, including subatomic particles, atomic models, valency, isotopes, and isobars. By practicing these questions, students can strengthen their understanding and excel in exams. For more educational resources, visit The Govt Guide.

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Authored by Rajesh Jaipal | The Govt Guide © 2025

My name is Rajesh Kumar, and I am from Gharsana, Sri Ganganagar in Rajasthan. I have completed my Bachelor of Science (B.Sc.) in 2017 and Bachelor of Education (B.Ed.) in 2019 from Maharaja Ganga Singh University, Bikaner. I hold a Master’s degree (M.A.) in Philosophy, completed in 2022, and I am currently pursuing M.A. in Sociology. I have work experience in the insurance sector and possess strong skills in MS Office, typing, and I hold a certificate in RSCIT. I am fluent in Hindi, Punjabi, and English. At present, I am preparing for competitive exams like DSSSB and UGC NET (Philosophy). I also run educational blog called “The Govt Guide” & "Literary Sphere", where I post content related to NCERT Notes, Govt job vacancy, literature and exam preparation. My hobbies include playing chess and reading books.