Where Mindful Learning Begins
Where Mindful Learning Begins
Complete NCERT Solutions for Chapter 8 of the new Class 9 Science Exploration textbook (CBSE 2026-27) — every Think It Over, Pause & Ponder, Think as a Scientist, What If, Inline Question, Ready to Go Beyond, Revise Reflect Refine, and Quest Continues question on this one page, with full step-by-step working for every numerical.
This chapter traces the historical journey from Dalton's solid-sphere atom to Thomson's plum pudding model, Rutherford's nuclear model, and finally Bohr's model with defined electron shells — each revised because new experimental evidence demanded it. You'll also work through atomic number, mass number, isotopes, isobars, and electron distribution using the 2n² rule, all core, heavily-tested topics in CBSE Class 9 Science Chapter 8 exams.
Journey Inside the Atom takes the atom apart, piece by piece — from Acharya Kanada's and Dalton's idea of an indivisible particle, through Thomson's plum pudding model, Rutherford's gold foil experiment and nuclear model, to Bohr's fixed energy shells. Along the way it introduces the electron, proton and neutron, and builds up to atomic number, mass number, isotopes and isobars. Every question is solved here, section by section, exactly as the textbook presents them, with full working for every numerical.
How each atomic model was built on — and eventually broke down against — new experimental evidence.
Why most alpha particles passed straight through gold foil, while a few bounced sharply back.
Working out protons, neutrons and electrons, and telling isotopes and isobars apart with numericals.
Subatomic particles
| Particle | Charge | Mass | Location |
|---|---|---|---|
| Proton | +1 | 1 u | Inside the nucleus |
| Neutron | 0 (neutral) | 1 u | Inside the nucleus |
| Electron | −1 | Negligible (~1/1836 u) | Orbiting the nucleus in shells |
Key formulae
In a neutral atom, the number of electrons always equals the number of protons. Maximum electrons in a shell = \(2n^2\), where \(n\) is the shell number (K=1, L=2, M=3...) — so K holds up to 2, L up to 8, M up to 18.
Isotopes vs. isobars
| Property | Isotopes | Isobars |
|---|---|---|
| Atomic number (Z) | Same | Different |
| Mass number (A) | Different | Same |
| Element | Same element | Different elements |
| Example | ¹H, ²H, ³H (protium, deuterium, tritium) | ¹⁴C and ¹⁴N |
Rutherford's gold-foil experiment showed that an atom's mass and positive charge are concentrated in a tiny, dense nucleus, while most of the atom is empty space — leading to the nuclear model of the atom.
Answer: no. Although ancient thinkers (Acharya Kanada, Leucippus, Democritus) and later Dalton (1808) proposed atoms as the smallest indivisible units of matter, 19th and 20th century experiments proved otherwise. The discovery of electrons by J. J. Thomson (1897), the nucleus by Rutherford (1911), and neutrons by Chadwick (1932) showed that atoms are made of smaller subatomic particles: electrons, protons, and neutrons. The phenomenon of radioactivity also proved atoms are divisible. Today we know atoms consist of a nucleus containing protons and neutrons, with electrons orbiting outside.
Answer: Rutherford's model could not explain this and was a key limitation. Niels Bohr resolved it in 1913 by proposing that electrons revolve in fixed circular paths called stationary states or shells (K, L, M, N…). In a stationary state, the electron has a definite constant energy and does not radiate or lose energy, even though it is moving. Therefore, it does not spiral inward into the nucleus. This is a postulate of Bohr's model — electrons can only exist in specific allowed orbits and nowhere in between.
A more complete explanation came from quantum mechanics (studied in higher grades), which describes electrons as existing in probability "clouds" around the nucleus.
Answer: scientists kept modifying atomic models because each new experiment revealed results that the existing model could not explain. This is how science progresses — through evidence-driven revision:
The willingness to revise models when new evidence emerges is the hallmark of scientific thinking.
(i) The model would represent a negatively charged ion, not a neutral atom. For a neutral atom, the total positive charge must exactly equal the total negative charge. If the clay (positive) carries less charge than the total bead charge (negative), the model has a net negative charge — like an anion.
(ii) No, the model would no longer represent a neutral atom. If the clay carries negative charge, both components — clay and beads — would be negative, making the total model negatively charged. It would fail to represent the positive-negative balance required for neutrality. Thomson's model specifically requires the positive sphere to balance the negative electrons.
Where it matches: both have a soft, distributed medium (pulp = positive sphere) with small embedded particles (seeds = electrons) scattered throughout, just as Thomson proposed electrons are embedded in the positive charge distribution.
Where it falls short: (1) the seeds in a fruit are not evenly distributed — they cluster near the centre, whereas Thomson proposed electrons spread throughout; (2) seeds are neutral objects, but electrons carry specific negative charge; (3) the pulp of the fruit is not electrically charged; (4) the number of seeds has no electrical relationship to the pulp, whereas in Thomson's model the electron charge must exactly balance the positive charge.
Answer: Thomson studied cathode rays in a cathode ray tube and found that (1) the rays were streams of negatively charged particles (electrons), and (2), crucially, the nature of cathode rays — their charge-to-mass ratio — was the same regardless of the material of the cathode used (iron, aluminium, copper, etc.) and regardless of the gas filled in the tube. Since identical particles were emitted from atoms of completely different elements under identical conditions, Thomson concluded that electrons must be a universal component present in every atom of every element — not specific to one material.
Alpha particles are positively charged. If replaced with negatively charged particles:
The key result of Rutherford's experiment (back-scattering) depended on electrostatic repulsion between like charges. With unlike charges, repulsion would become attraction and the experiment's logic would change completely.
Thomson's plum pudding model proposed that positive charge is spread uniformly throughout the atom like a diffuse cloud. In such a model, no small region of concentrated positive charge exists, so an alpha particle passing through would encounter only weak, distributed positive charge that could deflect it slightly at most — no point in the atom would be dense or strong enough to repel a fast, massive alpha particle backwards.
The back-scattering result completely rules this out because only a very small, extremely dense region of concentrated positive charge could exert a force strong enough to reverse the direction of a fast-moving, heavy, positively charged alpha particle. The very existence of even one bounced-back particle proves there must be a tiny, massive, positively charged nucleus — mathematically impossible under the Thomson model.
Answer: a strong scientific question would be: "Professor Rutherford, your experiment showed that the nucleus is extremely small and dense, but you could not explain why electrons don't spiral into it. Did you suspect at the time that the classical laws of physics might simply not apply inside the atom, or did you believe the answer would come from within classical mechanics?" This probes the conceptual leap from classical to quantum physics that Rutherford's work made necessary, and is the key unresolved question of his model.
This question has no single "correct" answer — it is open-ended and should demonstrate scientific curiosity and critical thinking about the limitations of Rutherford's model.
Correct answer: (ii) Both A and R are true, but R is not the correct explanation of A.
Assertion A is true — Rutherford did conclude from the gold foil experiment that most of an atom's mass and all its positive charge are concentrated in a small dense nucleus. Reason R is also true — Thomson's model does describe electrons embedded in a uniformly distributed positive sphere. However, R does not explain A: Rutherford's conclusion came from the gold foil scattering results (back-scattered alpha particles), not from Thomson's model — in fact, Rutherford's findings contradicted Thomson's model. The two statements are independently true but causally unrelated.
Example answer (student should use their own name): suppose the student's name is Arjun Sharma. The element could be named Arjunium (Aj). IUPAC rules followed: (1) first letter is capital — A is uppercase; (2) second letter is lowercase — j is lowercase; (3) symbol is derived from the English name of the element (Arjunium → Aj); (4) the symbol has two letters, which is standard.
Any student name applied correctly following the rules is acceptable. IUPAC rules: first letter uppercase; second letter (if any) lowercase; symbol derived from the element name (English, Latin, or other language); symbol must be internationally unique.
Standardised IUPAC symbols solve this by creating one universally recognised symbol per element, transcending language barriers worldwide.
Protons: atomic number (Z) = 26, so protons = 26. Electrons: atom is neutral, so electrons = protons = 26. Neutrons: mass number (A) = nucleons = 56, so neutrons = A − Z = 56 − 26 = 30.
Answer: Protons = 26, Electrons = 26, Neutrons = 30. This is Iron (Fe).
Answer: number of neutrons = 21. This element has Z = 20 (Calcium, Ca) with mass number 41.
Answer: mass number = 35. This is Chlorine-35 (³⁵Cl).
Mass number A = 23. Since the atom is neutral: protons = electrons = 11.
Answer: number of neutrons = 12. This is Sodium (Na), atomic number 11, mass number 23.
Answer: Sodium (Na). Z = 11 (protons = 11, electrons = 11). Mass number = 23.
Sodium has 12 neutrons. Its electronic configuration is 2, 8, 1. It is a soft silvery metal that reacts vigorously with water, releasing hydrogen gas and forming sodium hydroxide (NaOH).
Atomic numbers: both atoms have 11 protons, so both have atomic number (Z) = 11 — they are the same element, Sodium (Na).
Mass numbers: atom 1: A = 11 + 12 = 23. Atom 2: A = 11 + 13 = 24.
Answer: they are isotopes of sodium: Na-23 (more abundant, stable) and Na-24 (radioactive). Same element (same Z), different mass numbers. Same chemical properties, different physical properties.
Answer: the average atomic mass of bromine is approximately 79.9 u (≈ 80 u). This is the weighted average, reflecting that both isotopes occur in nearly equal proportions (unlike chlorine, where 75:25 gives 35.5 u).
With a thicker gold foil, there are more layers of gold atoms, so a beam of alpha particles encounters more nuclei in its path. Predicted observations:
Comparison: Thin foil → most straight through, few deflected. Thick foil → fewer straight through, many more deflected, more bounced back.
Atoms are mostly empty space — Rutherford showed the nucleus is about 10⁵ times smaller than the atom. If atoms had no empty space (i.e., the nucleus filled the entire atom):
Neutron stars are real examples of matter with very little empty space — they have the mass of the Sun compressed into a radius of ~10 km, with densities of ~10¹⁷ kg/m³.
From Table 8.4: Lithium (Li) has atomic number 3, 3 protons, 4 neutrons, and 3 electrons. Electronic configuration: K=2, L=1.
All three are isotopes of hydrogen — same atomic number (Z = 1), so same number of protons = 1, and since atoms are neutral, same number of electrons = 1.
Isotopes have the same atomic number (same electrons, same protons) but different numbers of neutrons.
Nitrogen in NH₃: nitrogen combines with 3 hydrogen atoms. Combining capacity of nitrogen = 3.
Magnesium in MgCl₂: magnesium combines with 2 chlorine atoms. Combining capacity of magnesium = 2.
Combining capacity (valency) is expressed in terms of the number of hydrogen or chlorine atoms that combine with one atom of the element.
Answer: no. Atoms with 8 electrons in the outermost shell (a complete octet) are already in a stable, lowest energy configuration. They have no tendency to gain, lose, or share electrons. These are the noble gases (He, Ne, Ar, Kr, Xe, Rn — except helium, which has 2 electrons filling its only shell). Noble gases are largely unreactive (inert) precisely because they already have a complete outer shell. This is why they exist as monoatomic gases and do not form compounds under normal conditions.
For each element, count the valence electrons and apply the rule — fewer than 4 valence electrons: lose that many electrons (valency = valence electrons); more than 4: gain electrons to complete the octet (valency = 8 − valence electrons); exactly 4: share (valency = 4). For example, Li (1 valence electron) has valency 1; O (6 valence electrons) has valency 2; C (4 valence electrons) has valency 4; F (7 valence electrons) has valency 1; Ne (8 valence electrons, complete octet) has valency 0.
Given: diameter of one atom ≈ 10⁻¹⁰ m. Thickness of paper = 0.1 mm = 10⁻⁴ m.
Answer: about one million atoms stacked together are needed to make a sheet of paper 0.1 mm thick. This illustrates how unimaginably tiny atoms are — a seemingly simple everyday object is actually a vast assembly of microscopic building blocks.
Mass numbers: X: 18+19=37, Y: 17+18=35, Z: 17+20=37
(i) Y and Z — isotopes: both Y and Z have the same atomic number (17) — they are both chlorine. But Y has mass number 35 and Z has mass number 37. Same element, different mass numbers → they are isotopes (³⁵Cl and ³⁷Cl).
(ii) Z and X — isobars: Z has mass number 37 (Z=17, Chlorine) and X has mass number 37 (Z=18, Argon). Same mass number but different atomic numbers → they are isobars.
From the back-scattering and large-angle deflection of a small fraction of alpha particles, Rutherford concluded:
Correct chronological order: (iv) → (ii) → (iii) → (i)
After Bohr: the quantum mechanical model was proposed (electrons exist as probability clouds, not fixed paths). Still being refined today.
Electrons do not fly away because of the electrostatic (Coulomb) force of attraction between the negatively charged electrons and the positively charged nucleus (protons). Opposite charges attract according to the fundamental law of electrostatics.
Correct answer: (ii) Both A and R are true, but R is not the correct explanation of A.
Assertion A is true — discovering electrons (Thomson, 1897), protons (Rutherford), and neutrons (Chadwick, 1932) was essential to understanding atomic structure — each discovery refined the atomic model. Reason R is also true — in a neutral atom, electrons = protons (equal opposite charges give neutrality). However, R does not explain A. The equality of electrons and protons is a property of neutral atoms derived from the discoveries, not the reason why those discoveries aided understanding.
(i) Protons: atomic number (Z) = 12, so protons = 12. (ii) Neutrons: Neutrons = Mass number − Protons = 24 − 12 = 12. (iii) Electrons: neutral atom, so electrons = protons = 12.
Electronic configuration (2, 8, 2): K-shell = 2, L-shell = 8, M-shell = 2.
Diagram: draw 3 concentric circles around a nucleus marked "+12". Place 2 electrons on the K-shell (innermost), 8 on the L-shell (middle), 2 on the M-shell (outermost). The M-shell with 2 electrons is the valence shell → valency of Mg = 2 (loses 2 electrons).
Read each diagram by counting the electrons in each shell, then work out identity and valency:
Valency: He has a full first shell (2 electrons) → inert, valency 0. B loses 3 → valency 3. P gains 3 → valency 3. Mg loses 2 → valency 2.
Rutherford's failure: in classical physics (which Rutherford used), a charged particle moving in a circular path continuously accelerates (changes direction). An accelerating charged particle must radiate electromagnetic energy. So an orbiting electron should continuously lose energy, spiral inward, and collapse into the nucleus in a fraction of a second (~10⁻⁸ s). Rutherford's model had no mechanism to prevent this, so it predicted atoms should be unstable — contradicting observable reality.
Bohr's solution: Bohr introduced a radical new postulate — electrons in certain specific circular orbits (stationary states) do NOT radiate energy. In these allowed orbits, the electron's energy remains constant despite its motion. This was not explained by classical physics — it was a new quantum rule. Electrons can only jump between these fixed orbits by absorbing or emitting a specific quantum of energy equal to the energy difference between levels. Since electrons can only exist in these fixed states (not spiral continuously), atoms are stable.
Mass number (A) = 70. Since the atom is neutral: protons = electrons = 31.
Answer: neutrons = 39. This atom is Gallium (Ga), atomic number 31, mass number 70.
(i) Neutrons: Neutrons = Mass number − Protons = 197 − 79 = 118. (ii) Electrons: neutral atom, so electrons = protons = 79.
Answer: neutrons = 118, electrons = 79. This is Gold (Au), atomic number 79.
Working:
(i) Protons and electrons: Protons = Mass number − Neutrons = 35 − 18 = 17. Neutral atom → electrons = 17.
(ii) Atomic number: Z = number of protons = 17
(iii) Identify element X: Z = 17 → Chlorine (Cl)
(iv) Electronic configuration: Z = 17: K=2, L=8, M=7. Configuration: 2, 8, 7
(v) Valence electrons: outermost shell (M) has 7 electrons → 7 valence electrons
(vi) Mass number if 2 neutrons added: new neutrons = 18+2 = 20. New mass number = 17+20 = 37
(vii) Relation of X with the new atom: both have Z=17 (Chlorine) but different mass numbers (35 and 37) — they are isotopes, ³⁵Cl and ³⁷Cl, the two naturally occurring isotopes of chlorine.
(i) Atomic number: atomic number = number of protons = 12. The hypothetical particles replace electrons (not protons), so the number of protons is unchanged. Atomic number remains 12.
(ii) Atomic mass: atomic mass includes protons + neutrons + electrons (though electrons are negligible normally). With 12 hypothetical particles each 500× heavier than an electron: added mass = 12 × 500 × electron mass ≈ 12 × 500 × 9.1×10⁻³¹ kg ≈ 5.46×10⁻²⁷ kg, which is comparable to a proton's mass (~1.67×10⁻²⁷ kg). Atomic mass increases noticeably — roughly by about 3–4 u compared to the original.
(iii) Mass number: mass number = total nucleons (protons + neutrons in the nucleus) = 12+12 = 24. Mass number is defined as nucleons only — the hypothetical particles are not nucleons. Mass number stays 24.
(iv) Overall charge: the hypothetical particles have the same charge as electrons (−1 each), and there are 12 of them, same as 12 protons (+1 each). The charges still balance → overall charge = 0. The atom remains electrically neutral.
This is one of the deepest questions in science, and the honest answer is: not yet, and possibly never completely. Here is why:
Conclusion: we have an extraordinarily powerful and accurate model (quantum mechanics), but complete understanding of everything inside an atom may be fundamentally limited by the nature of quantum reality itself. The journey of discovery continues — and that is what makes physics exciting.
Each atomic model — Dalton's indivisible sphere, Thomson's plum pudding, Rutherford's tiny nucleus, Bohr's fixed energy shells — was built to explain what the previous model could not, and the atom's true structure of protons, neutrons and electrons, arranged as atomic number, mass number, isotopes and isobars, is the product of that century-long chain of revision.
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