This MCQ module is based on: Journey Inside the Atom — NCERT Exercises
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Journey Inside the Atom — NCERT Exercises
🎓 Class 9
Science
CBSE
Theory
Ch 8 — Journey Inside the Atom
⏱ ~13 min
🌐 Language: [gtranslate]
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Chapter Summary — Journey Inside the Atom
Chapter 8 traces the gradual journey from "the atom is indivisible" to "the atom is built from smaller particles". The big ideas of the chapter are summarised below.
Atom is divisible
Cathode-ray and canal-ray experiments proved that atoms contain still smaller charged particles.
Three subatomic particles
Electron (−1, ≈0 mass), proton (+1, 1 u), neutron (0, 1 u).
Dalton (1808)
Atom = solid indivisible sphere. Explained laws of chemical combination but not electricity.
Thomson (1898)
Plum-pudding model — positive sphere with embedded electrons.
Rutherford (1911)
α-scattering revealed a tiny dense nucleus; the rest of the atom is empty space.
Bohr (1913)
Electrons revolve in fixed energy shells (K, L, M, N) without radiating energy.
Z and A
Atomic number = protons. Mass number = protons + neutrons. Notation \( {}^{A}_{Z}\text{X} \).
Isotopes & Isobars
Isotopes: same Z, different A. Isobars: different Z, same A.
2n² Rule
Maximum electrons in shell n = 2n². Outermost shell ≤ 8.
Valency
Combining capacity = number of valence electrons (or 8 − that number).
Keyword Glossary
AtomSmallest unit of an element that retains its chemical identity, made of subatomic particles.
ElectronNegatively charged particle (−1), mass ≈ 1/1836 u; located outside nucleus.
ProtonPositively charged particle (+1), mass ≈ 1 u; located in nucleus.
NeutronNeutral particle (0), mass ≈ 1 u; located in nucleus.
Cathode raysStreams of electrons emitted from cathode in a discharge tube.
Canal raysPositive rays travelling through holes in a perforated cathode.
NucleusTiny dense positively-charged core containing protons and neutrons.
Atomic number (Z)Number of protons in the nucleus; defines the element.
Mass number (A)Total number of protons and neutrons in the nucleus.
IsotopesSame Z, different A — same element, different mass.
IsobarsSame A, different Z — different elements, same mass.
Shell / OrbitFixed energy path around the nucleus along which electrons revolve.
Valence electronsElectrons in the outermost shell; decide chemistry.
Octet ruleAtoms tend to gain/lose/share electrons to attain 8 in their outermost shell.
ValencyCombining capacity of an atom.
NCERT Exercises (15 Questions with Solutions)
Click "Show Solution" to reveal the answer to each question.
Q1. Compare the properties of electrons, protons and neutrons.
| Property | Electron | Proton | Neutron |
|---|---|---|---|
| Symbol | e⁻ | p⁺ | n |
| Charge | −1 | +1 | 0 |
| Relative mass (u) | ≈ 1/1836 | 1 | 1 |
| Location | Shells outside nucleus | Nucleus | Nucleus |
| Discoverer | J.J. Thomson | E. Goldstein | J. Chadwick |
Q2. What are the limitations of J.J. Thomson's model of the atom?
Thomson's plum-pudding model could not explain Rutherford's α-scattering experiment. With a uniformly spread positive charge, large-angle deflections of α-particles are impossible. The model also did not predict the existence of a nucleus, and gave no information about the arrangement of electrons or atomic stability.
Q3. What are the limitations of Rutherford's model of the atom?
According to classical electromagnetic theory, an electron revolving in a circular orbit must continuously radiate energy. It would therefore lose energy, spiral inward, and eventually collide with the nucleus — making the atom unstable. But atoms are clearly stable. Rutherford's model could not explain this.
Q4. Describe Bohr's model of the atom.
(i) An atom has a small dense positively charged nucleus containing protons and neutrons. (ii) Electrons revolve around the nucleus only in certain permitted orbits called shells (K, L, M, N, …) of definite energy. (iii) While in any one of these orbits an electron does not radiate energy, so the atom is stable. (iv) An electron jumping from a higher to a lower shell emits a fixed packet of energy as light; jumping the other way absorbs the same packet.
Q5. Compare all the proposed models of the atom in a table.
| Model | Picture of atom | Stability of atom | Explained α-scattering? |
|---|---|---|---|
| Dalton | Indivisible solid sphere | Not addressed | No (could not foresee experiments) |
| Thomson | Positive sphere with embedded electrons | Not addressed | No |
| Rutherford | Tiny nucleus + electrons in orbits | Predicts unstable atom | Yes |
| Bohr | Nucleus + electrons in fixed shells | Stable | Yes |
Q6. Summarise the rules for writing the distribution of electrons in various shells of an atom.
(i) Maximum electrons in shell number n = \(2n^{2}\). So K = 2, L = 8, M = 18, N = 32. (ii) The outermost shell can hold a maximum of 8 electrons. (iii) The next-to-outermost shell cannot have more than 18. (iv) Electrons fill from the innermost shell outward — a new shell starts only after the previous shell can hold no more (subject to rules ii and iii).
Q7. Define valency by taking examples of silicon and oxygen.
Valency is the combining capacity of an atom — the number of electrons it can lose, gain, or share to attain a stable octet.
Silicon (Z = 14): configuration 2, 8, 4. With 4 valence electrons it shares 4, hence valency = 4.
Oxygen (Z = 8): configuration 2, 6. With 6 valence electrons it gains 2 to complete the octet, hence valency = 8 − 6 = 2.
Silicon (Z = 14): configuration 2, 8, 4. With 4 valence electrons it shares 4, hence valency = 4.
Oxygen (Z = 8): configuration 2, 6. With 6 valence electrons it gains 2 to complete the octet, hence valency = 8 − 6 = 2.
Q8. Explain with examples (a) atomic number, (b) mass number, (c) isotopes, (d) isobars. Give one use each of isotopes.
(a) Atomic number (Z) = number of protons. Sodium has Z = 11.
(b) Mass number (A) = number of protons + neutrons. Sodium has A = 23.
(c) Isotopes: atoms of the same element with different mass numbers — \({}^{12}\)C and \({}^{14}\)C.
(d) Isobars: atoms of different elements with the same mass number — \({}^{40}\)Ar and \({}^{40}\)Ca.
Uses of isotopes: \({}^{60}\)Co — cancer treatment; \({}^{131}\)I — treatment of goitre; \({}^{235}\)U — fuel in nuclear reactors; \({}^{14}\)C — radio-carbon dating.
(b) Mass number (A) = number of protons + neutrons. Sodium has A = 23.
(c) Isotopes: atoms of the same element with different mass numbers — \({}^{12}\)C and \({}^{14}\)C.
(d) Isobars: atoms of different elements with the same mass number — \({}^{40}\)Ar and \({}^{40}\)Ca.
Uses of isotopes: \({}^{60}\)Co — cancer treatment; \({}^{131}\)I — treatment of goitre; \({}^{235}\)U — fuel in nuclear reactors; \({}^{14}\)C — radio-carbon dating.
Q9. Na⁺ has completely filled K and L shells. Explain.
Sodium (Z = 11) has configuration 2, 8, 1. The lone M-shell electron is loosely held. Sodium loses it to form Na⁺, leaving 10 electrons distributed as 2, 8 — both K and L shells completely filled (octet achieved, like neon).
Q10. If bromine atom is available in the form of two isotopes \({}^{79}_{35}\text{Br}\) (49.7%) and \({}^{81}_{35}\text{Br}\) (50.3%), calculate the average atomic mass of bromine.
Average mass = \(\dfrac{49.7 \times 79 + 50.3 \times 81}{100} = \dfrac{3926.3 + 4074.3}{100} = \dfrac{8000.6}{100} \approx 80.0\) u.
Q11. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes \({}^{16}_{8}\text{X}\) and \({}^{18}_{8}\text{X}\) in the sample?
Let the percentage of \({}^{16}\)X be p, then \({}^{18}\)X = (100 − p).
\(\dfrac{16p + 18(100-p)}{100} = 16.2\) ⇒ \(16p + 1800 - 18p = 1620\) ⇒ \(-2p = -180\) ⇒ \(p = 90\).
Therefore \({}^{16}\)X = 90% and \({}^{18}\)X = 10%.
\(\dfrac{16p + 18(100-p)}{100} = 16.2\) ⇒ \(16p + 1800 - 18p = 1620\) ⇒ \(-2p = -180\) ⇒ \(p = 90\).
Therefore \({}^{16}\)X = 90% and \({}^{18}\)X = 10%.
Q12. If Z = 3, what would be the valency of the element? Also, name the element.
Z = 3 ⇒ configuration 2, 1. The outermost shell has 1 electron, hence valency = 1. The element is lithium (Li).
Q13. Composition of nuclei of two atomic species X and Y are given. X has 6 protons and 6 neutrons; Y has 6 protons and 8 neutrons. Find the mass numbers and the relation between the two species.
Mass number of X = 6 + 6 = 12. Mass number of Y = 6 + 8 = 14. Both have the same atomic number (Z = 6, carbon) but different mass numbers — therefore X (\({}^{12}\)C) and Y (\({}^{14}\)C) are isotopes of carbon.
Q14. For the symbol H, D and T tabulate three subatomic particles found in each of them.
| Symbol | Name | Protons | Neutrons | Electrons |
|---|---|---|---|---|
| \({}^{1}_{1}\)H | Protium | 1 | 0 | 1 |
| \({}^{2}_{1}\)D | Deuterium | 1 | 1 | 1 |
| \({}^{3}_{1}\)T | Tritium | 1 | 2 | 1 |
Q15. Write the electronic configurations of any one pair of isotopes and isobars.
Isotope pair: \({}^{35}_{17}\)Cl and \({}^{37}_{17}\)Cl. Both have Z = 17 and the same configuration 2, 8, 7.
Isobar pair: \({}^{40}_{18}\)Ar (Z = 18, configuration 2, 8, 8) and \({}^{40}_{20}\)Ca (Z = 20, configuration 2, 8, 8, 2). Both have A = 40 but different Z, hence different configurations.
Isobar pair: \({}^{40}_{18}\)Ar (Z = 18, configuration 2, 8, 8) and \({}^{40}_{20}\)Ca (Z = 20, configuration 2, 8, 8, 2). Both have A = 40 but different Z, hence different configurations.
Self-check: Once you can write the configuration of any of the first 20 elements without looking, predict its valency, name its likely ions, and identify its isotopes/isobars when given Z and A — you have mastered Chapter 8.
Frequently Asked Questions — NCERT Exercises & Intext Questions
How do I solve NCERT Class 9 Science Chapter 8 (Journey Inside the Atom) exercise questions for the CBSE board exam?
Solve NCERT Chapter 8 — Journey Inside the Atom — exercise questions by first reading the question carefully, writing down the given data, recalling the relevant concepts like electron, proton, neutron, and applying them step by step. This Part 4 covers every intext and end-of-chapter exercise from the NCERT textbook. Write balanced equations, label diagrams clearly and show each step — CBSE Class 9 examiners award step marks even if the final answer has a small slip. Practising these solutions strengthens conceptual clarity and builds speed for both the school exam and the upcoming Class 10 board exam.
Are the NCERT intext questions from Journey Inside the Atom important for the Class 9 Science exam?
Yes, NCERT intext questions for Chapter 8 Journey Inside the Atom are highly important for the CBSE Class 9 Science exam. Many questions in school and competitive papers are directly lifted or only slightly modified from these intext questions, and they test the foundational concepts — electron, proton, neutron — that chapter-end questions and the Class 10 board build on. Attempt every intext question first, then move on to the exercises. This practice ensures complete NCERT coverage, which is the CBSE syllabus's primary source.
What types of questions from Journey Inside the Atom are asked in the Class 9 Science exam?
The Class 9 Science paper (CBSE pattern) asks a mix of question types from Journey Inside the Atom: 1-mark MCQ and assertion-reason, 2-mark short answers, 3-mark explanations, 5-mark long answers with diagrams or derivations, and 4-mark competency-based / case-study questions. These test understanding of electron, proton, neutron, atomic models. Practising every NCERT exercise and intext question prepares you to answer all of these formats with confidence.
How many marks does Chapter 8 — Journey Inside the Atom — typically carry in the Class 9 Science paper?
Chapter 8 — Journey Inside the Atom — is part of the CBSE Class 9 Science syllabus and typically contributes 5–9 marks in the annual paper, depending on the year's weightage. Questions are drawn from definitions, reasoning, numerical/descriptive problems and diagrams on topics like electron, proton, neutron. Solving the NCERT exercises in this part is essential because CBSE directly references the NCERT Exploration textbook for question design.
Where can I find step-by-step NCERT solutions for Chapter 8 Journey Inside the Atom Class 9 Science?
You can find complete, step-by-step NCERT solutions for Chapter 8 Journey Inside the Atom Class 9 Science on MyAiSchool. Every intext and end-of-chapter exercise question is solved with full working, labelled diagrams and CBSE-aligned mark distribution. Solutions highlight key points about electron, proton, neutron that examiners look for. This makes revision quick and exam-focused for Class 9 CBSE students.
What is the best way to revise Journey Inside the Atom for the Class 9 Science exam?
The best way to revise Journey Inside the Atom for the CBSE Class 9 Science exam is a three-pass approach. First pass: skim the chapter and note down key terms like electron, proton, neutron in a one-page mind map. Second pass: solve every NCERT intext and exercise question without looking at the solution, then self-check. Third pass: attempt sample papers and competency-based questions under timed conditions. This structured revision secures full marks for this chapter.
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