Journey Inside the Atom — Class 9 MCQs with Answers

Journey Inside the Atom — important questions & MCQs with answers (Class 9 Science)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Which subatomic particle carries a positive charge?

   AElectronBPhotonCNeutronDProton
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   Answer: 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. Q2 of 6Easy

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

   AThomson modelBRutherford modelCDalton modelDBohr model
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   Answer: 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. Q3 of 6Medium

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

   A18B8C2D32
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   Answer: 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.

4. Q4 of 6Medium

   Assertion (A): Neutrons add to the mass of an atom but not to its charge. Reason (R): The neutron was discovered by James Chadwick in 1932.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true but R is not the correct explanation of A

   Explanation: Both are true. A is true because a neutron has a mass nearly equal to that of a proton but no electrical charge. Who discovered the neutron, and when, does not explain why it adds mass but no charge.

5. Q5 of 6Hard

   Thomson's model assumed positive charge is:

   ASpread uniformly through the atomBConcentrated at the centreCConfined to electronsDOutside the atom
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   Answer: Spread uniformly through the atom

   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.

6. Q6 of 6Hard

   Assertion (A): An atom with mass number 35 and 18 neutrons has a valency of 1. Reason (R): An atom with 7 valence electrons gains one electron to complete its octet.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true and R is the correct explanation of A

   Explanation: Both are true. The atom has 35 − 18 = 17 protons, so it is chlorine with configuration 2, 8, 7. With 7 valence electrons it gains one electron to complete its octet, so its valency is 1.

Journey Inside the Atom notes: key concepts (Class 9 Science)

Atoms are made of protons, neutrons and electrons, and successive models refined how those parts are arranged. This chapter follows the models from Thomson to Bohr, explains electronic configuration and valency, and distinguishes isotopes from isobars.

Subatomic particles

The electron carries a negative charge and negligible mass, the proton a positive charge, and the neutron no charge but a mass like the proton's.

Thomson's model

The atom pictured as a sphere of positive charge with electrons embedded in it, like seeds in a watermelon; it explained neutrality but not much else.

Rutherford's alpha-scattering experiment

Most alpha particles passed straight through gold foil while a very few rebounded, showing the atom is mostly empty with a tiny dense positive centre.

Rutherford's model and its failure

Electrons orbit a small nucleus, but classical physics says an accelerating electron must radiate energy and spiral inward, making the atom unstable.

Bohr's model

Electrons revolve only in certain permitted discrete orbits, or shells, without radiating energy — which resolved the stability problem.

Energy shells

Shells labelled K, L, M, N outward from the nucleus, each with a fixed energy; an electron absorbs or releases energy only when it jumps between them.

Bohr-Bury rules for filling shells

A shell holds at most 2n² electrons, the outermost holds at most 8, and shells fill in order from the innermost outwards.

Electronic configuration

The distribution of an atom's electrons among its shells, written as 2, 8, 8 and so on, which determines chemical behaviour.

Valence electrons and valency

Electrons in the outermost shell; valency is the combining capacity, being the electrons gained, lost or shared to complete an octet.

Atomic number

The number of protons in the nucleus, written Z; it identifies the element and equals the number of electrons in a neutral atom.

Mass number

The total number of protons and neutrons, written A; electrons are excluded because their mass is negligible.

Isotopes

Atoms of the SAME element with the same atomic number but different mass numbers, differing only in neutron count.

Isobars

Atoms of DIFFERENT elements having the same mass number but different atomic numbers.

Applications of isotopes

Uranium-235 as nuclear fuel, cobalt-60 in cancer treatment and iodine-131 in treating goitre.