Introduction

NCERT Solutions for Class 9 Science Chapter 8, Journey Inside the Atom, help students understand the structure of atoms and the developments that led to our modern understanding of atomic structure. This chapter from the new NCERT textbook, Exploration: A Textbook of Science for Grade 9, introduces the fundamental particles that make up an atom and explains how scientists developed models to describe its structure.

The solutions provide clear explanations for textbook questions and activities, helping students understand atomic structure, compare different atomic models, and prepare effectively for examinations.

Chapter Overview

Class 9 Science Chapter 8 – Journey Inside the Atom

This chapter takes students on a journey into the structure of matter and explores what lies inside an atom. Students learn about the development of ideas regarding atomic structure and the experiments that helped scientists identify subatomic particles.

The chapter introduces concepts related to electrons, protons, and neutrons, along with different models proposed to explain how these particles are arranged within an atom.

Students also learn how scientific models change as new experimental evidence becomes available.

Class 9 Science Chapter 8 Important Topics

Include the concepts actually covered in your current NCERT edition, such as:

• Structure of the atom
• Subatomic particles
• Electrons
• Protons
• Neutrons
• Discovery of subatomic particles
• Atomic models
• Thomson’s model
• Rutherford’s model
• Limitations of atomic models
• Development of atomic theory
• Atomic structure and scientific evidence

NCERT Intext Solution

(For exact question see NCERT Textbook)

1. Question

(i) The positive charge on the clay is lesser than the total negative charge of the beads?
→ The atom would have an overall negative charge because the negative charge would be greater than the positive charge. It would no longer be a neutral atom.

(ii) By mistake, the clay itself carries a bit of negative charge? Would your model still represent a neutral atom?
→ No. The atom would not be neutral unless the total positive and negative charges are exactly equal. Extra negative charge on the clay would make the atom negatively charged.

2. Question

Matches Thomson’s idea:

• The soft pulp can represent the positively charged material.
• The seeds can represent electrons embedded in it.
• Seeds are spread throughout the pulp, similar to electrons in Thomson’s model.

Falls short:

• Seeds are much larger than electrons.
• The fruit does not represent electrical charges.
• Electrons are extremely tiny and have specific properties, unlike seeds.
• The distribution of seeds in a fruit is not exactly like the distribution of electrons in an atom.

3. Question

→ Thomson observed that cathode rays were produced regardless of the type of gas used or the material of the electrodes. Since the same negatively charged particles were always obtained, he concluded that these particles (electrons) are common to all atoms and are a fundamental component of matter.

4. Question

→ Negatively charged particles would be attracted towards the positively charged nucleus. As a result, many particles would be deflected more strongly, and some might even be captured by the atom. The pattern of scattering would be very different from that observed with positively charged α-particles.

5. Question

→ According to Thomson’s model, positive charge was spread uniformly throughout the atom. Therefore, α-particles should have passed through with only small deflections. The fact that a few α-particles bounced back sharply showed that the atom contains a tiny, dense, positively charged nucleus capable of strongly repelling α-particles. This observation could not be explained by Thomson’s model, so it was rejected.

6. Question

Sample Answer:
“How did you feel when you first observed some α-particles bouncing back from the gold foil, and did you immediately realize that it meant the atom had a nucleus?”

(Any thoughtful question related to his experiment is acceptable.)

7.Question

Answer: (ii) Both A and R are true, but R is not the correct explanation of A.

Explanation:

Assertion (A): Rutherford concluded that most of the mass of an atom is concentrated in a small region at the centre called the nucleus.
True

Reason (R): According to Thomson’s model, electrons are embedded in a uniformly distributed positive charge sphere.
True

However, the reason only describes Thomson’s atomic model and does not explain why Rutherford concluded that mass is concentrated in the nucleus. Rutherford’s conclusion came from the gold foil experiment, where a few α-particles were deflected sharply or bounced back.

Correct Option: (ii) Both A and R are true, but R is not the correct explanation of A.

8. Question

Answer: If I discovered a new element, I could name it Tusharium (after Tushar).

Symbol: Tu

Justification:

• The symbol consists of one capital letter followed by one lowercase letter.
• It is derived from the element’s name.
• It follows the IUPAC convention for writing element symbols.

9. Question

Answer: If different scientists used different symbols for the same element:

1. There would be confusion in scientific communication.
2. Scientists from different countries might misinterpret chemical formulas and equations.
3. Research findings would become difficult to understand and compare.
4. Textbooks and scientific records would lack uniformity.
5. International collaboration in science would become more difficult.

Therefore, a standard system of symbols (IUPAC) is essential for clear and universal communication. ✅

10. Question

• Atomic number (Z) = 26
• Mass number (A) = 56

Protons = 26
Electrons = 26 (for a neutral atom)
Neutrons = A − Z = 56 − 26 = 30

Answer:

• Electrons = 26
• Protons = 26
• Neutrons = 30

11. Question

• Protons = 20
• Mass number = 41

Neutrons = 41 − 20 = 21

Answer: 21 neutrons

12. Question

• Atomic number = 17 (Protons = 17)
• Neutrons = 18

Mass number = Protons + Neutrons = 17 + 18 = 35

Answer: 35

13. Question

• Mass number = 23
• Electrons = 11

For a neutral atom:

Protons = Electrons = 11

Neutrons = Mass number − Protons = 23 − 11 = 12

Answer: 12 neutrons

14. Question

(i) (Carbon)
Electronic configuration = 2, 4
Outermost shell electrons = 4

(ii) (Fluorine)
Electronic configuration = 2, 7
Outermost shell electrons = 7

(iii)  (Silicon)
Electronic configuration = 2, 8, 4
Outermost shell electrons = 4

15. Question

16. Question

• Mass number = 23
• Number of protons = 11

Since atomic number = number of protons = 11, the element is Sodium (Na).

Number of neutrons = Mass number − Atomic number

= 23 − 11

= 12

Answer:

• Element: Sodium (Na)
• Number of neutrons: 12

17. Question

• Number of protons = 11 for both atoms.
• Therefore, their atomic number = 11 (same for both).

Mass number = Protons + Neutrons

• First atom:
• Second atom:

Answer:

• Atomic numbers: Same (11)
• Mass numbers: Different (23 and 24)
• They are the same element (Sodium) but are different isotopes.

18. Question

Average atomic mass

Answer:

Average atomic mass of bromine = 80.006 u (≈ 80.0 u).

NCERT Exercise Solution

(For exact question see NCERT Textbook)

1. Question

(i) Incorrect

Reason: Rutherford’s gold foil experiment did not discover neutrons. Neutrons were discovered later by James Chadwick (1932).

(ii) Correct

Reason: Large-angle deflections of a few α-particles showed that positive charge is concentrated in a small central region, disproving Thomson’s plum pudding model.

(iii) Correct

Reason: Rutherford concluded that almost all the mass and positive charge are concentrated in a very small region called the nucleus.

(iv) Incorrect

Reason: Rutherford’s experiment provided evidence only for the existence of the nucleus. It did not explain the arrangement of electrons around it.

2. Question

(i) Incorrect

Reason: According to Bohr, electrons in fixed orbits do not lose energy.

(ii) Incorrect

Reason: Electrons occupy only specific orbits with definite energies.

(iii) Correct

Reason: These stable paths are called stationary orbits.

(iv) Incorrect

Reason: Electrons cannot exist between two energy levels; they jump from one level to another by absorbing or emitting energy.

3. Question

(i) Y and Z

Number of protons = 17 (same)

Mass numbers:

• Y = 17 + 18 = 35
• Z = 17 + 20 = 37

Y and Z are isotopes because they have the same atomic number but different mass numbers.

(ii) Z and X

Mass numbers:

• Z = 17 + 20 = 37
• X = 18 + 19 = 37

Atomic numbers are different (17 and 18).

Z and X are isobars because they have the same mass number but different atomic numbers.

4. Question

Rutherford concluded that:

• The positive charge and almost all the mass of an atom are concentrated in a very small, dense central region called the nucleus.
• The nucleus is positively charged.
• Most of the atom is empty space.
• Alpha particles that came very close to the nucleus experienced strong repulsion and were deflected or bounced back.

Final Answers (in brief)

1. (i) ✗, (ii) ✓, (iii) ✓, (iv) ✗

2. (i) ✗, (ii) ✗, (iii) ✓, (iv) ✗

3.

• (i) Y and Z → Isotopes
• (ii) Z and X → Isobars

4. Positive charge and most of the mass are concentrated in a tiny, dense, positively charged nucleus at the centre of the atom, while most of the atom is empty space.

5. Question

Correct order of evolution of atomic models:

1. (iv) Dalton’s model described atoms as indivisible particles.
2. (ii) Thomson’s model proposed the “plum pudding” model.
3. (iii) Rutherford’s model proposed a dense central nucleus.
4. (i) Bohr’s model proposed fixed orbits with definite energies.

Chronological order:

(iv) → (ii) → (iii) → (i)

6. Question

Electrons are negatively charged and the nucleus contains positively charged protons. The electrostatic force of attraction between opposite charges keeps electrons bound to the nucleus. According to Bohr’s model, electrons revolve in fixed energy levels and do not lose energy while moving in these orbits.

7. Question

Answer:

(ii) Both A and R are true, but R is not the correct explanation of A.

Explanation: The discovery of electrons, protons, and neutrons led to a better understanding of atomic structure. Although the number of electrons equals the number of protons in a neutral atom, this fact does not explain the assertion.

8. Question

(i) Number of protons

= Atomic number = 12

(ii) Number of neutrons

= Mass number – Atomic number
= 24 – 12 = 12

(iii) Number of electrons

= Number of protons = 12

Arrangement of electrons

K shell = 2 electrons
L shell = 8 electrons
M shell = 2 electrons

Electronic configuration = 2, 8, 2

9. Question

Electronic configurations:

• (a) He: 2
• (b) N: 2, 5
• (c) Al: 2, 8, 3
• (d) O: 2, 6

These are the answers according to the Bohr models shown in Fig. 8.17.

10. Question

Rutherford’s model failed because according to classical physics, revolving electrons should continuously lose energy and eventually fall into the nucleus, making the atom unstable.

Bohr’s model succeeded because it proposed that electrons revolve in fixed energy levels (orbits) without losing energy. Hence, atoms remain stable.

11. Question

Mass number = 70

Number of electrons = Number of protons = 31

Number of neutrons = Mass number – Number of protons = 70 – 31 = 39

Answer: 39 neutrons

12. An atom has 79 protons and a mass number of 197. Calculate:

(i) Number of neutrons = 197 – 79 = 118

(ii) Number of electrons = Number of protons = 79

Answer: Neutrons = 118, Electrons = 79

13. Question

14. Question

(i) Number of protons and electrons

Protons = 35 – 18 = 17

Electrons = 17

(ii) Atomic number = Number of protons

= 17

(iii) Identify the element X

Atomic number 17 corresponds to Chlorine (Cl).

(iv) Electronic configuration 2, 8, 7

(v) Number of valence electrons

= 7

(vi) Mass number if two neutrons are added

New mass number = 35 + 2

= 37

(vii) Relation of X with the new atom

The new atom has the same atomic number but different mass number.

Therefore, they are isotopes of each other.

15. Question

(i) Atomic number

Atomic number depends only on the number of protons.

Atomic number = 12 (unchanged)

(ii) Atomic mass

Since the new particles are much heavier than electrons, the atomic mass will increase.

(iii) Mass number

Mass number = Number of protons + Number of neutrons = 12 + 12 = 24

Hence, mass number remains unchanged.

(iv) Overall charge

Since the replacing particles have the same negative charge and are equal in number to electrons, the atom remains electrically neutral. Overall charge = Zero (neutral)

Common Mistakes to Avoid in Chapter 8

Students should avoid:

• Confusing the charges of electrons and protons.
• Forgetting that neutrons have no net charge.
• Mixing up different atomic models.
• Confusing experimental observations with conclusions.
• Writing incomplete explanations of Rutherford’s experiment.
• Drawing incorrectly labelled atomic models.
• Memorising model names without understanding why each model was proposed.
• Ignoring the limitations of earlier models.

12. FAQs

FAQs on Class 9 Science Chapter 8

What is Chapter 8 of Class 9 Science?

Chapter 8 of the new Class 9 Science textbook is Journey Inside the Atom. It introduces students to atomic structure, subatomic particles, and the development of models used to explain the atom.

What are the important topics in Chapter 8?

Important topics include the structure of atoms, subatomic particles, their properties, atomic models, experimental evidence, and the development of scientific ideas about atomic structure.

What are the three main subatomic particles?

The three commonly discussed subatomic particles are electrons, protons, and neutrons. They differ in charge and other properties.

What is an atomic model?

An atomic model is a scientific representation used to explain the structure and behaviour of an atom. Different models were developed as scientists obtained new experimental evidence.

Why are atomic models important?

Atomic models help scientists explain observations about matter and atomic structure. They can be modified or replaced when new evidence shows that an earlier model is incomplete.

How should I prepare Class 9 Science Chapter 8?

Read the NCERT chapter carefully, understand the experiments and observations, study the atomic models and diagrams, and practise the textbook questions. Use NCERT Solutions to check your answers and clarify difficult concepts.

Are NCERT Solutions enough for Chapter 8?

NCERT Solutions are useful for checking textbook questions, but students should also read the complete NCERT chapter and understand the experiments, diagrams, observations, and explanations.

Is Chapter 8 important for the Class 9 Science exam?

Yes. Students should understand the structure of the atom, properties of subatomic particles, atomic models, and the experimental evidence discussed in the chapter.