Structure of the Atom — Class 9 MCQs with Answers

Structure of the Atom — important questions & MCQs with answers (Class 9 Science)

14 solved questions from this chapter's difficulty levels, each with its answer and explanation. The other 76 are timed and scored when you sign in.

1. Q1Easy

   Which subatomic particle carries a positive charge?

   A.Electron

   B.Photon

   C.Neutron

   D.Proton✓ Correct

   Answer: D. Proton

   Explanation: Protons carry +1 charge, electrons −1, neutrons 0 — protons are the only positive particle in an atom. It's easy to mistake the neutral neutron for the positive one.

2. Q2Easy

   The model in which electrons revolve in fixed circular orbits is the:

   A.Thomson model

   B.Rutherford model

   C.Dalton model

   D.Bohr model✓ Correct

   Answer: D. Bohr model

   Explanation: Bohr's model placed electrons in fixed, quantised circular orbits (energy shells). Rutherford's earlier model had no such fixed orbits, which is the usual confusion.

3. Q3Easy

   Energy shells around the nucleus are labelled in order as:

   A.K, L, M, N✓ Correct

   B.A, B, C, D

   C.1, 2, 3, 4 only

   D.X, Y, Z, W

   Answer: A. K, L, M, N

   Explanation: Energy shells are named K, L, M, N for n=1,2,3,4 outward from the nucleus — letters, not numbers or other letter sets.

4. Q4Easy

   Which particles are found in the nucleus?

   A.Electrons and protons

   B.Protons and neutrons✓ Correct

   C.Only neutrons

   D.Only electrons

   Answer: B. Protons and neutrons

   Explanation: The nucleus holds protons and neutrons (nucleons); electrons instead orbit around it in shells. Placing electrons inside the nucleus is a common mix-up.

5. Q5Easy

   The 'plum pudding' model of the atom was proposed by:

   A.Bohr

   B.Rutherford

   C.Thomson✓ Correct

   D.Dalton

   Answer: C. Thomson

   Explanation: Thomson's 'plum pudding' model pictured the atom as a sphere of positive charge with electrons studded through it. This predates and differs from Rutherford's nuclear model.

6. Q6Easy

   The outermost shell of an atom is also called the:

   A.Core shell

   B.Valence shell✓ Correct

   C.K shell

   D.Inner shell

   Answer: B. Valence shell

   Explanation: The outermost occupied shell is the valence shell, holding the valence electrons that govern bonding. It is not the innermost K shell, a common mix-up.

7. Q7Medium

   Atomic number of an element is equal to:

   A.Number of neutrons

   B.Number of protons✓ Correct

   C.Number of electrons + protons

   D.Mass number

   Answer: B. Number of protons

   Explanation: Atomic number (Z) is the number of protons in the nucleus, and equals the electron count in a neutral atom. Mass number instead adds neutrons on top of that.

8. Q8Medium

   A drawback of Rutherford's atomic model was that:

   A.It did not explain origin of mass

   B.It violated Dalton's theory

   C.It denied existence of nucleus

   D.Electrons in circular orbits should radiate and spiral into the nucleus✓ Correct

   Answer: D. Electrons in circular orbits should radiate and spiral into the nucleus

   Explanation: Classically, an accelerating charge radiates energy continuously, and orbiting electrons are always accelerating, so they should spiral into the nucleus. Rutherford's model gave no reason this doesn't happen — Bohr's quantised orbits fixed it.

9. Q9Medium

   Maximum number of electrons that can be accommodated in the K shell (n=1) is:

   A.18

   B.8

   C.2✓ Correct

   D.32

   Answer: C. 2

   Explanation: Shell capacity follows 2n²; for the K shell, n=1, giving a maximum of 2×1²=2 electrons — the smallest capacity of any shell.

10. Q10Medium

    Atoms of the same element with different mass numbers are called:

    A.Isotopes✓ Correct

    B.Isobars

    C.Isotones

    D.Allotropes

    Answer: A. Isotopes

    Explanation: Isotopes are atoms of the same element (same atomic number) with different mass numbers, because their neutron counts differ. Isobars instead share mass number, not atomic number.

11. Q11Medium

    Thomson's atomic model failed to explain:

    A.Existence of electrons

    B.Mass of an atom

    C.Scattering of α-particles by atoms✓ Correct

    D.Existence of protons

    Answer: C. Scattering of α-particles by atoms

    Explanation: Thomson's spread-out positive charge predicted only gentle deflection of alpha particles. It couldn't explain the sharp, large-angle scattering that needed a small, dense nucleus.

12. Q12Hard

    An atom with 11 protons and 10 electrons has a charge of:

    A.−1

    B.0

    C.+1✓ Correct

    D.+2

    Answer: C. +1

    Explanation: Net charge = protons − electrons = 11 − 10 = +1, a cation like Na⁺. More protons than electrons always gives a positive charge.

13. Q13Hard

    Thomson's model assumed positive charge is:

    A.Distributed uniformly throughout the atom with electrons embedded in it✓ Correct

    B.Concentrated at the centre

    C.Confined to electrons

    D.Outside the atom

    Answer: A. Distributed uniformly throughout the atom with electrons embedded in it

    Explanation: Thomson pictured the atom as a uniform sphere of positive charge with electrons scattered through it, with no concentrated centre — unlike the later nuclear model's tiny, dense nucleus.

14. Q14Hard

    The valency of an element with electronic configuration 2, 8, 1 is:

    A.7

    B.2

    C.3

    D.1✓ Correct

    Answer: D. 1

    Explanation: An outer shell with just 1 electron (configuration 2,8,1, as in sodium) is easiest satisfied by losing that electron to reach a stable octet, giving valency 1.

Structure of the Atom — FAQs

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