CBSE Class 9 Science Chapter 8: Journey Inside the Atom — Assertion & Reason
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The following Assertion and Reason questions are based on Chapter 8: Journey Inside the Atom from the NCERT Class 9 Science textbook. Each question has four options - read both the Assertion (A) and Reason (R) carefully before selecting your answer.
How to attempt Assertion & Reason questions:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is NOT the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is NOT the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Assertion & Reason Questions
1
Assertion (A): Early ideas of parmanu and atomos were philosophical rather than experimentally established models.
Reason (R): Dalton later proposed a scientific atomic theory based on experimental evidence available in his time.
Reason (R): Dalton later proposed a scientific atomic theory based on experimental evidence available in his time.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. The transition from speculative ideas to evidence-based theory marks a major stage in the development of atomic science.
2
Assertion (A): Acharya Kanada described indivisible particles called parmanus.
Reason (R): Greek thinkers used the word atomos for indivisible particles.
Reason (R): Greek thinkers used the word atomos for indivisible particles.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both historical statements are correct, but the Greek term does not explain Kanada's proposal.
3
Assertion (A): Thomson's model pictured electrons embedded in a diffuse positively charged sphere.
Reason (R): Thomson's model placed all positive charge inside a tiny central nucleus.
Reason (R): Thomson's model placed all positive charge inside a tiny central nucleus.
Answer: (c)
A is true, but R is false. The concentrated nucleus was introduced after Rutherford's scattering evidence.
4
Assertion (A): Rutherford's gold foil experiment showed that most alpha particles were reflected backward.
Reason (R): Most alpha particles passed through the foil with little or no deflection.
Reason (R): Most alpha particles passed through the foil with little or no deflection.
Answer: (d)
A is false, but R is true. The dominant straight-through result indicated that most of the atom is empty space.
5
Assertion (A): The large deflection of a very small fraction of alpha particles indicated a tiny, massive, positively charged nucleus.
Reason (R): Strong deflection requires encountering a concentrated region of positive charge and mass.
Reason (R): Strong deflection requires encountering a concentrated region of positive charge and mass.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. A diffuse positive charge could not account for the rare large-angle scattering events.
6
Assertion (A): Rutherford's model described the atom as mostly empty space.
Reason (R): James Chadwick later discovered the neutron.
Reason (R): James Chadwick later discovered the neutron.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both statements are correct, but the discovery of the neutron does not explain Rutherford's conclusion about empty space.
7
Assertion (A): Rutherford proposed that electrons move around a central nucleus.
Reason (R): His model fully explained atomic stability by assigning electrons fixed energy shells.
Reason (R): His model fully explained atomic stability by assigning electrons fixed energy shells.
Answer: (c)
A is true, but R is false. The fixed-energy-shell explanation was a later feature of Bohr's model.
8
Assertion (A): In Bohr's model, electrons may occupy any energy while moving anywhere between shells.
Reason (R): Bohr proposed fixed energy levels or shells for electrons around the nucleus.
Reason (R): Bohr proposed fixed energy levels or shells for electrons around the nucleus.
Answer: (d)
A is false, but R is true. Electrons are restricted to permitted energy levels in the model.
9
Assertion (A): Neutrons are electrically neutral particles located in the nucleus.
Reason (R): Together with protons, neutrons account for nearly all of an atom's mass.
Reason (R): Together with protons, neutrons account for nearly all of an atom's mass.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. Electrons contribute very little to atomic mass compared with the nucleons.
10
Assertion (A): An electron carries negative charge, whereas a proton carries positive charge.
Reason (R): A neutron has approximately the same mass as a proton but no electric charge.
Reason (R): A neutron has approximately the same mass as a proton but no electric charge.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both particle descriptions are true, but the neutron's properties do not explain the opposite charges of electrons and protons.
11
Assertion (A): The atomic number of an element equals the number of protons in its nucleus.
Reason (R): In every ion, the number of electrons must equal the number of protons.
Reason (R): In every ion, the number of electrons must equal the number of protons.
Answer: (c)
A is true, but R is false. Electron gain or loss creates ions, while proton number and atomic number remain unchanged.
12
Assertion (A): Mass number is the total number of electrons and neutrons in an atom.
Reason (R): Mass number equals the total number of protons and neutrons in the nucleus.
Reason (R): Mass number equals the total number of protons and neutrons in the nucleus.
Answer: (d)
A is false, but R is true. Electrons are not counted as nucleons in the mass number.
13
Assertion (A): In standard nuclide notation, the lower number gives atomic number and the upper number gives mass number.
Reason (R): Atomic number counts protons, while mass number counts protons plus neutrons.
Reason (R): Atomic number counts protons, while mass number counts protons plus neutrons.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. The two definitions determine the placement and meaning of the numbers in the symbol.
14
Assertion (A): The K shell can hold at most two electrons, while the L shell can hold up to eight.
Reason (R): Electrons fill available shells in an ordered manner when atoms are built up.
Reason (R): Electrons fill available shells in an ordered manner when atoms are built up.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both statements are correct, but the general filling order does not by itself explain the numerical capacities.
15
Assertion (A): An atom with electronic configuration 2, 8, 7 generally has valency one.
Reason (R): It becomes stable most readily by gaining seven additional electrons.
Reason (R): It becomes stable most readily by gaining seven additional electrons.
Answer: (c)
A is true, but R is false. It needs only one more electron to complete its outer octet.
16
Assertion (A): An atom with a complete outer octet is usually highly reactive.
Reason (R): A complete outer shell generally gives a stable and largely unreactive electronic configuration.
Reason (R): A complete outer shell generally gives a stable and largely unreactive electronic configuration.
Answer: (d)
A is false, but R is true. Atoms tend to gain, lose, or share electrons to reach such stable arrangements.
17
Assertion (A): Isotopes of an element have the same atomic number but different mass numbers.
Reason (R): They contain the same number of protons but different numbers of neutrons.
Reason (R): They contain the same number of protons but different numbers of neutrons.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. Changing neutron number changes mass number without changing elemental identity.
18
Assertion (A): Isotopes of an element usually have similar chemical properties.
Reason (R): Their physical properties and masses can differ because their neutron numbers differ.
Reason (R): Their physical properties and masses can differ because their neutron numbers differ.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both statements are true, but the physical differences do not explain the chemical similarity, which follows from similar electron arrangements.
19
Assertion (A): The average atomic mass of an element depends on the masses and natural abundances of its isotopes.
Reason (R): It is always the simple arithmetic mean of isotope masses regardless of their abundances.
Reason (R): It is always the simple arithmetic mean of isotope masses regardless of their abundances.
Answer: (c)
A is true, but R is false. A weighted average must account for how common each isotope is.
20
Assertion (A): Isobars are atoms of the same element with the same atomic number but different mass numbers.
Reason (R): Isobars are atoms of different elements that share a mass number but have different atomic numbers.
Reason (R): Isobars are atoms of different elements that share a mass number but have different atomic numbers.
Answer: (d)
A is false, but R is true. The assertion defines isotopes, whereas the reason correctly defines isobars.
❓ Frequently Asked Questions
What is covered in CBSE Class 9 Science Chapter 8 Journey Inside the Atom?
This chapter covers all key topics from Journey Inside the Atom as per CBSE 2026-27 syllabus.
Is this Assertion & Reason useful for CBSE board exams?
Yes, designed for CBSE Class 9 board exam preparation covering the complete syllabus.
Are these CBSE Class 9 Science Assertion & Reason updated for 2026-27?
Yes, all content at eBookPublisher is updated as per the latest 2026-27 CBSE syllabus.
How many chapters are in CBSE Class 9 Science?
All chapters of CBSE Class 9 Science are covered at eBookPublisher with free Assertion & Reason for each chapter.
Can I study Journey Inside the Atom online for free?
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