Chapter 8, “Journey Inside the Atom,” takes Class 9 Science students from Dalton’s indivisible atom all the way to Bohr’s model of fixed energy levels, covering electrons, protons, neutrons, atomic number, mass number, isotopes, isobars, valency, and electronic configuration. These solutions were prepared by working through every official “Revise, Reflect, Refine” exercise question and every in-text question (Think It Over, Pause and Ponder, Think as a Scientist, What If…?, and more) from the new NCERT “Exploration” edition, cross-checking answers and numerical working across LearnCBSE, Vedantu, and Boundless Maths before finalising them here.
Last Updated: September 23, 2026
NCERT Solutions for Class 9 Science Chapter 8: Journey Inside the Atom
Revise, Reflect, Refine (NCERT Textbook, Page No. 158)
1. Choose the correct options and explain the reason for the correct and incorrect options in the context of Ernest Rutherford’s gold foil experiment:
(i) The experiment clearly showed the existence of neutrons in the nucleus.
(ii) The results disproved the plum pudding model and led to the idea of a nucleus at the centre of the atom.
(iii) The large deflection of a few alpha particles indicated that most of the mass of the atom and positive charge are packed into a tiny centre.
(iv) The way alpha particles were deflected showed that electrons move around the nucleus.
(i) Incorrect. The gold foil experiment did not reveal neutrons at all; neutrons were discovered later by James Chadwick in 1932.
(ii) Correct. The unexpected deflections could not be explained by Thomson’s plum pudding model, so Rutherford proposed a small, dense, positively charged nucleus at the centre of the atom.
(iii) Correct. Only a very small fraction of alpha particles bounced back sharply, showing that a tiny central region carries nearly all the mass and the entire positive charge.
(iv) Incorrect. The experiment gave no information about how electrons are arranged or how they move; that idea came later with Bohr’s model.
2. Which of the following statements are correct or incorrect according to Bohr’s atomic model? Give a reason for each statement.
(i) Electrons lose energy while moving in fixed orbits and slowly fall into the nucleus.
(ii) Electrons can exist anywhere around the nucleus with no fixed energy.
(iii) Electrons revolve around the nucleus in orbits of fixed energy without losing energy.
(iv) Electrons can be found between energy levels as they move around the nucleus.
(i) Incorrect. Bohr proposed that electrons moving in permitted (fixed) orbits do not radiate or lose energy, so they do not spiral into the nucleus.
(ii) Incorrect. Electrons can occupy only specific, quantised energy levels, not any arbitrary position around the nucleus.
(iii) Correct. This is the central postulate of Bohr’s model — electrons revolve in orbits of fixed, definite energy without losing energy.
(iv) Incorrect. Electrons cannot exist in between two energy levels; they jump directly from one permitted orbit to another.
3. The composition of the nuclei of three atomic species X, Y, and Z are given below. Explain the relation between the following:
(i) Y and Z
(ii) Z and X
X — protons 18, neutrons 19 (mass number 37); Y — protons 17, neutrons 18 (mass number 35); Z — protons 17, neutrons 20 (mass number 37).
(i) Y and Z have the same number of protons (17) but a different number of neutrons (18 and 20), so they are isotopes of the same element.
(ii) Z and X have different numbers of protons (17 and 18) but the same mass number (37, since 17+20 = 18+19), so they are isobars.
4. What conclusion did Rutherford draw about the position and characteristics of the atom’s positively charged part based on the few alpha particles that bounced back or were deflected at large angles in the gold foil experiment?
Rutherford concluded that the positive charge and most of the mass of an atom are concentrated in an extremely small, dense region at its centre, which he called the nucleus. Since only a very small fraction of alpha particles were deflected sharply, he also inferred that the nucleus occupies a tiny fraction of the atom’s total volume, with the rest being largely empty space.
5. Explain and arrange the following statements in the correct chronological order to show how atomic models have evolved over time.
(i) Bohr’s model proposed that electrons move in fixed orbits around the nucleus, each with a definite energy.
(ii) Thomson’s model depicted the atom as a ‘plum pudding’ with electrons embedded in a sphere of positive charge.
(iii) Rutherford’s model proposed that atoms have a dense central nucleus.
(iv) Dalton’s model described atoms as indivisible particles.
Correct chronological order: (iv) Dalton’s model (atoms are indivisible) → (ii) Thomson’s model (plum pudding, electrons embedded in positive matter) → (iii) Rutherford’s model (dense central nucleus, mostly empty space) → (i) Bohr’s model (electrons in fixed energy orbits).
6. Electrons move around the nucleus in orbits. Why do they not fly away from the atom? Explain what keeps them attracted to the nucleus.
The nucleus carries a positive charge (due to protons) while electrons carry a negative charge. The electrostatic force of attraction between these opposite charges pulls the electrons toward the nucleus and keeps them bound in their orbits, preventing them from flying off.
7. Assertion (A): The discovery of subatomic particles helped in understanding the atomic structure.
Reason (R): The number of electrons is equal to the number of protons in an atom.
Choose the correct option:
(i) Both A and R are true, and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.
Correct option: (ii) Both A and R are true, but R is not the correct explanation of A. It is true that discovering electrons, protons, and neutrons helped explain atomic structure, and it is separately true that in a neutral atom the number of electrons equals the number of protons — but the second fact does not explain why discovering subatomic particles was useful for understanding atomic structure.
8. Magnesium is essential for many biological processes, including muscle contraction. For an atom of magnesium with a mass number of 24 and atomic number 12, determine the number of (i) protons, (ii) neutrons, (iii) electrons, and also illustrate the arrangement of electrons in a magnesium atom.
(i) Number of protons = atomic number = 12.
(ii) Number of neutrons = mass number − number of protons = 24 − 12 = 12.
(iii) Number of electrons = number of protons (neutral atom) = 12.
Electronic configuration of magnesium (12 electrons) = 2, 8, 2 — the first shell (K) holds 2, the second shell (L) holds 8, and the third shell (M) holds the remaining 2.
9. Find the following information for the elements shown in Fig. 8.17: (i) Name of the element (ii) Symbol (iii) Total number of electrons (iv) Number of valence electrons (v) Valency of the element (vi) Number of protons (vii) Atomic number
(a) Lithium (Li): total electrons 3, configuration 2,1 → valence electrons 1, valency 1, protons 3, atomic number 3.
(b) Nitrogen (N): total electrons 7, configuration 2,5 → valence electrons 5, valency 3 (8 − 5), protons 7, atomic number 7.
(c) Aluminium (Al): total electrons 13, configuration 2,8,3 → valence electrons 3, valency 3, protons 13, atomic number 13.
(d) Fluorine (F): total electrons 9, configuration 2,7 → valence electrons 7, valency 1 (8 − 7), protons 9, atomic number 9.
10. Both Rutherford’s and Bohr’s models have electrons orbiting the nucleus. Why did Rutherford’s model fail to explain atomic stability, while Bohr’s model succeeded?
In Rutherford’s model, orbiting electrons — being charged particles undergoing continuous acceleration — should, according to classical electromagnetic theory, continuously radiate energy, lose speed, and spiral into the nucleus, making the atom unstable. This is not observed in reality. Bohr overcame this problem by proposing that electrons revolve only in certain fixed, permitted orbits of definite energy in which they do not radiate energy, so they do not lose energy or spiral inward — this explains why atoms are stable.
11. An atom X-70 has 31 electrons. How many neutrons are there in its nucleus?
Mass number = 70; number of electrons = 31, so number of protons = 31 (neutral atom).
Number of neutrons = mass number − number of protons = 70 − 31 = 39.
12. An atom has 79 protons and a mass number of 197. Calculate (i) the number of neutrons, and (ii) the number of electrons.
(i) Number of neutrons = mass number − number of protons = 197 − 79 = 118.
(ii) Number of electrons = number of protons = 79 (this corresponds to gold, Au).
13. Complete the Table 8.5 (Atomic number, Mass number, Number of neutrons, Number of protons, Number of electrons, Name of the element):
Atomic number 5, mass number 11 → neutrons 6, protons 5, electrons 5 — Boron.
Atomic number 7, mass number 14 → neutrons 7, protons 7, electrons 7 — Nitrogen.
Atomic number 12, mass number 24 → neutrons 12, protons 12, electrons 12 — Magnesium.
Atomic number 15, mass number 31 → neutrons 16, protons 15, electrons 15 — Phosphorus.
Atomic number 1, mass number 1 → neutrons 0, protons 1, electrons 1 — Hydrogen.
14. Aman was discussing the structure of an atom with his classmates. During the discussion, he learnt that an element X has a mass number of 35 and contains 18 neutrons. Based on this information, answer the following: (i) How many electrons and protons does element X have? (ii) What is its atomic number? (iii) Identify the element, X. (iv) Write its electronic configuration. (v) How many valence electrons does it have? (vi) What will be the mass number if two neutrons are added to its nucleus? (vii) What will be the relation of X with the new atom?
(i) Number of protons = mass number − number of neutrons = 35 − 18 = 17; number of electrons = number of protons = 17.
(ii) Atomic number = 17.
(iii) Element X is Chlorine (Cl).
(iv) Electronic configuration = 2, 8, 7.
(v) Valence electrons = 7.
(vi) New mass number = 35 + 2 = 37.
(vii) The new atom (Cl-37) and the original atom (Cl-35) have the same atomic number but different mass numbers, so they are isotopes of chlorine.
15. In an atom, there are 12 protons and 12 neutrons in the nucleus. Now imagine that all the electrons are replaced with some hypothetical particles that have the same charge as electrons but are 500 times heavier. What effect will this replacement have on the atom’s: (i) Atomic number (ii) Atomic mass (iii) Mass number (iv) Overall charge?
(i) Atomic number stays the same (12) because it depends only on the number of protons, which is unchanged.
(ii) Atomic mass increases, because the new particles replacing the electrons are 500 times heavier, adding significantly more mass than ordinary (near-negligible-mass) electrons would.
(iii) Mass number stays the same (24) because it is defined as the sum of protons and neutrons only — it does not include electrons (or their heavier replacements) at all.
(iv) The overall charge remains neutral, since the number of unit negative charges (from the heavier particles) still equals the number of protons (12 positive charges).
In-Text Questions (Think It Over, Pause and Ponder, Think as a Scientist, What If…?, Inline Questions, Ready to Go Beyond, The Quest Continues)
Think It Over — Are atoms the smallest indivisible particles? (Page No. 140)
No. Atoms were once believed to be the smallest indivisible particles of matter (as Dalton proposed), but later experiments showed that atoms are themselves made up of smaller subatomic particles — electrons, protons, and neutrons.
Think It Over — Why do electrons not fall into the nucleus even though they are attracted to protons in it? (Page No. 140)
Electrons move in fixed, permitted energy levels around the nucleus. According to Bohr’s model, electrons revolving in these orbits do not lose energy, so they remain in stable orbits instead of spiralling inward and collapsing into the nucleus.
Think It Over — Why did scientists keep modifying atomic models? (Page No. 140)
As new experiments were performed, they produced observations that earlier models could not explain. Scientists revised and refined their models each time new evidence demanded a better explanation, leading to progressively more accurate pictures of the atom.
Pause and Ponder — Suppose you made up your own ‘atom’ using clay for the positive charge and small beads for the electrons spread through it. What will happen if (i) the positive charge on the clay is lesser than the total negative charge of the beads? (ii) the clay itself carries a bit of negative charge by mistake — would your model still represent a neutral atom? (Page No. 143)
(i) If the positive charge on the clay is less than the total negative charge of the beads, the overall model would carry a net negative charge and would not be neutral.
(ii) If the clay itself carries some negative charge in addition to the beads, the total negative charge would exceed the positive charge, and the model would again fail to represent a neutral atom.
Pause and Ponder — Could an orange or a lemon, which also contains seeds inside soft pulp, be a good comparison to Thomson’s model? In what ways does it match, and where does it fall short? (Page No. 143)
It matches Thomson’s model in that the soft pulp can represent the sphere of positive charge and the seeds can represent the embedded electrons. However, it falls short because Thomson’s model assumes the positive charge is spread perfectly uniformly through the sphere, whereas in a real orange or lemon the seeds are not evenly distributed and tend to cluster in particular regions of the pulp.
Pause and Ponder — Why did Thomson conclude that electrons are present in all atoms? (Page No. 143)
Thomson observed that cathode rays (streams of electrons) were produced no matter what gas or which metal was used as the cathode, and the particles always had the same charge-to-mass characteristics. Since the result did not depend on the material used, he concluded that electrons must be a fundamental constituent present in the atoms of every element.
Pause and Ponder — What do you think would happen if alpha particles were replaced with negatively charged particles in Rutherford’s gold foil experiment? (Page No. 144)
Negatively charged particles would be attracted toward the positively charged nucleus rather than repelled by it. As a result, they would not show the large-angle deflections or sharp bounce-backs seen with alpha particles; instead, most would be pulled toward and could even be captured near the nucleus.
Pause and Ponder — Rutherford found that a few alpha particles bounced back sharply. How does this single surprising result completely rule out Thomson’s ‘plum pudding’ model of the atom? (Page No. 144)
In Thomson’s model, positive charge is spread thinly and uniformly throughout the atom, so no single region could exert enough repulsive force to reverse the path of a fast, heavy alpha particle. The sharp bounce-back observed by Rutherford could only happen if the positive charge (and most of the mass) were concentrated in a very small, dense region — which directly contradicts Thomson’s assumption of uniformly spread charge.
Pause and Ponder — If you could ask Rutherford one question about his work, what would it be? (Page No. 144)
This is an open-ended, reflective question. A good example: “How exactly are the electrons arranged around the nucleus, and what stops them from losing energy and falling into it?” — a question Rutherford’s own model could not answer, and which was later addressed by Bohr.
Think as a Scientist — Observe the gold foil experiment. Predict the observations you would expect if the gold foil were made thicker, and sketch a simple diagram to show them. (Page No. 144)
A thicker foil has more layers of gold atoms, so alpha particles are more likely to pass near or collide with a nucleus. This means fewer particles would pass straight through undeflected, while more particles would be scattered at various angles, and a greater proportion would be deflected sharply or bounce back compared to a thin foil.
Pause and Ponder — Assertion (A): Rutherford concluded that most of the mass of an atom is concentrated in a small region at the centre called the nucleus. Reason (R): According to Thomson’s model, electrons are embedded in a uniformly distributed positive charge sphere. Choose the correct option. (Page No. 145)
Correct option: Both A and R are true, but R is not the correct explanation of A. Rutherford’s conclusion about the nucleus came from analysing the results of his own gold foil experiment, not from Thomson’s model — in fact, his results contradicted Thomson’s model rather than being explained by it.
What If…? — What if an atom had no empty space? How would this have affected the size of various objects? (Page No. 147)
Since atoms are mostly empty space (with an extremely small, dense nucleus and electrons occupying a much larger volume around it), removing that empty space would make matter enormously more compact. If atoms had no empty space, all objects — including our own bodies — would shrink to a minute fraction of their current size.
Pause and Ponder — Imagine you are a scientist who has discovered a new element. Name this element after yourself and justify that the symbol you have chosen follows the IUPAC rules. (Page No. 149)
Sample answer: naming the element “Amanium” with the symbol Am follows IUPAC convention because element symbols use one or two letters derived from the element’s name, with only the first letter capitalised.
Pause and Ponder — What problems could arise if every scientist used different symbols for the same element? (Page No. 149)
If different scientists used different symbols for the same element, it would cause confusion in writing chemical formulas and equations, make scientific communication across countries and languages difficult, and prevent a universal, standardised system of naming — which is exactly why IUPAC rules for element symbols were established.
Inline Question — Observe the atomic structure diagram of lithium. How many protons and neutrons does the lithium atom shown have?
Lithium has atomic number 3 and a mass number of 7 (its most common isotope), so it has 3 protons and 7 − 3 = 4 neutrons.
Inline Question — Hydrogen exists in three isotopic forms — protium, deuterium, and tritium. How many electrons does each of these have?
All three isotopes of hydrogen have the same atomic number (1), so each has exactly 1 electron and 1 proton in the neutral atom. They differ only in the number of neutrons: protium has 0 neutrons, deuterium has 1 neutron, and tritium has 2 neutrons.
Inline Question — What is the combining capacity (valency) of nitrogen in NH₃ and of magnesium in MgCl₂?
In NH₃, one nitrogen atom combines with three hydrogen atoms, so the valency of nitrogen is 3. In MgCl₂, one magnesium atom combines with two chlorine atoms, so the valency of magnesium is 2.
Inline Question — Do atoms with a complete octet (a fully filled outermost shell of 8 electrons) still react chemically?
No. Atoms with a complete octet in their outermost shell — like the noble gases — have a highly stable electronic configuration and show very little or no tendency to react with other atoms under normal conditions.
Inline Question — Add a valency column to the table of elements and their electronic configurations, and explain how you worked it out.
Valency can be found from the number of valence (outermost-shell) electrons: if the outermost shell has 1, 2, 3, or 4 electrons, that number is usually the valency; if it has 5, 6, or 7 electrons, the valency is generally 8 minus that number, since the atom tends to gain electrons to complete its octet. For example, sodium (2, 8, 1) has valency 1; oxygen (2, 6) has valency 8 − 6 = 2; and chlorine (2, 8, 7) has valency 8 − 7 = 1.
Pause and Ponder — An atom with an atomic number of 26 has 56 nucleons. Find its number of electrons, protons, and neutrons. (Page No. 150)
Atomic number = 26, so protons = 26 and electrons = 26.
Number of neutrons = total nucleons − number of protons = 56 − 26 = 30. (This corresponds to iron, Fe-56.)
Pause and Ponder — The nucleus of an atom contains 20 protons. If its mass number is 41, find the number of neutrons in it. (Page No. 150)
Number of neutrons = mass number − number of protons = 41 − 20 = 21. (This corresponds to calcium, Ca-41.)
Pause and Ponder — An atom has 18 neutrons and an atomic number of 17. What is its mass number? (Page No. 150)
Number of protons = atomic number = 17.
Mass number = number of protons + number of neutrons = 17 + 18 = 35. (This corresponds to chlorine, Cl-35.)
Pause and Ponder — An atom A-23 has 11 electrons. Find the number of neutrons in it. (Page No. 150)
Number of protons = number of electrons = 11.
Number of neutrons = mass number − number of protons = 23 − 11 = 12. (This corresponds to sodium, Na-23.)
Pause and Ponder — Identify the number of electrons in the outermost shell of the following: (i) Carbon (atomic number 6, mass number 12) (ii) Fluorine (atomic number 9, mass number 19) (iii) Silicon (atomic number 14, mass number 28). (Page No. 152)
(i) Carbon: 6 electrons, configuration 2, 4 → 4 electrons in the outermost shell.
(ii) Fluorine: 9 electrons, configuration 2, 7 → 7 electrons in the outermost shell.
(iii) Silicon: 14 electrons, configuration 2, 8, 4 → 4 electrons in the outermost shell.
Pause and Ponder — Write the electronic configuration of the elements having atomic numbers 12, 16, and 18. (Page No. 152)
Atomic number 12 (Magnesium): 2, 8, 2.
Atomic number 16 (Sulphur): 2, 8, 6.
Atomic number 18 (Argon): 2, 8, 8.
Pause and Ponder — Solve this riddle: I am an atom with a mass number of 23 and 11 protons. I am a soft metal and react vigorously with water. Who am I, and how many neutrons do I have? (Page No. 152)
An atom with 11 protons has atomic number 11, which is sodium (Na) — a soft metal that reacts vigorously with water.
Number of neutrons = mass number − number of protons = 23 − 11 = 12. So the element is sodium, with 12 neutrons.
Pause and Ponder — Two different atoms have 11 protons each, but one has 12 neutrons and the other has 13 neutrons. How do their atomic numbers and mass numbers compare? Are they the same element or different elements? (Page No. 156)
Both atoms have the same atomic number (11), since atomic number depends only on the number of protons. Their mass numbers differ: 11 + 12 = 23 for the first, and 11 + 13 = 24 for the second. Since they have the same atomic number but different mass numbers, they are isotopes of the same element (sodium), not different elements.
Pause and Ponder — If a bromine atom is available in the form of two isotopes, Br-79 (49.7%) and Br-81 (50.3%), calculate the average atomic mass of bromine. (Page No. 156)
Average atomic mass = (79 × 49.7)/100 + (81 × 50.3)/100.
79 × 49.7 = 3926.3, and 81 × 50.3 = 4074.3.
Sum = 3926.3 + 4074.3 = 8000.6.
Average atomic mass = 8000.6 ÷ 100 = 80.006 u.
Ready to Go Beyond — Roughly how many atoms thick is an ordinary sheet of paper, if the paper is about 0.1 mm thick and a typical atom’s diameter is of the order of 10⁻¹⁰ m?
Paper thickness = 0.1 mm = 1 × 10⁻⁴ m. Atomic diameter ≈ 1 × 10⁻¹⁰ m.
Number of atoms stacked across the thickness = (1 × 10⁻⁴) ÷ (1 × 10⁻¹⁰) = 1 × 10⁶.
So a sheet of paper is roughly a million atoms thick — a striking illustration of just how small an atom is.
The Quest Continues — Is it possible to completely understand everything that happens inside an atom?
Not entirely — even after protons, neutrons, and electrons were discovered, scientists found that protons and neutrons are themselves made of smaller particles called quarks, and that the behaviour of particles at this scale is governed by quantum mechanics, where properties like an electron’s exact position and energy cannot both be known with complete certainty at the same time. This means our understanding of the atom keeps deepening with new research, but it may never be absolutely “complete.”
Why This Chapter Matters
“Journey Inside the Atom” builds directly on the idea of elements, compounds, and particles of matter introduced in earlier chapters, and it lays the essential groundwork for everything that follows in chemistry — from the periodic classification of elements to chemical bonding and reactions in later classes. A firm grip on atomic number, mass number, isotopes, isobars, valency, and electronic configuration here makes topics like writing chemical formulas, balancing equations, and understanding the periodic table far easier to grasp in Class 10 and beyond, which is why this chapter is considered one of the most foundational in the Class 9 Science syllabus.
Extra Questions | Revision Notes | Formulas Handbook
Chapter Quiz — Test Your Understanding
Class 9 Science Chapter 8 – Notes and Extra Questions
Along with these NCERT Solutions, students can also use the Class 9 Science Chapter 8 Extra Questions and Class 9 Science Chapter 8 Revision Notes for quick revision and extra practice.
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Frequently Asked Questions
How many questions are there in the Class 9 Science Chapter 8 “Revise, Reflect, Refine” exercise?
There are 15 questions in the official “Revise, Reflect, Refine” exercise on page 158 of the NCERT Exploration textbook, covering atomic models, subatomic particles, isotopes, isobars, valency, and numerical problems based on atomic number and mass number.
What is the difference between isotopes and isobars?
Isotopes are atoms of the same element that have the same atomic number (same number of protons) but a different number of neutrons, giving them different mass numbers — for example, Cl-35 and Cl-37. Isobars are atoms of different elements that happen to have the same mass number but different atomic numbers — for example, the atoms with 18 protons/19 neutrons and 17 protons/20 neutrons discussed in Question 3 of the exercise, both with mass number 37.
Why did Bohr’s atomic model succeed where Rutherford’s model failed?
Rutherford’s model could not explain why electrons, which continuously accelerate while orbiting the nucleus, do not lose energy and spiral into it. Bohr resolved this by proposing that electrons move only in specific, fixed-energy orbits in which they do not radiate energy, which explained why atoms remain stable.
How do you work out the valency of an element from its electronic configuration?
Find the number of electrons in the outermost (valence) shell. If this number is 1, 2, 3, or 4, it is usually the valency itself. If it is 5, 6, or 7, the valency is generally 8 minus that number, because the atom tends to gain electrons to complete its outermost shell (octet). Elements with a complete octet (8 valence electrons), like noble gases, generally have zero valency.
What is the maximum number of electrons a shell can hold, and how is it used in this chapter?
Each electron shell can hold a maximum of 2n² electrons, where n is the shell number counted from the nucleus (K=1, L=2, M=3, and so on). This rule is used throughout the chapter to work out electronic configurations, such as 2, 8, 2 for magnesium (atomic number 12) or 2, 8, 8 for argon (atomic number 18).
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