Important Questions Class 9 Science Chapter 8 cover Journey Inside the Atom as per the CBSE 2026 syllabus. An atom is the smallest unit of matter that retains the chemical properties of an element.
Students will find class 9 science chapter 8 important questions with answers on electrons, protons, neutrons, Thomson’s model, Rutherford’s model, Bohr’s model, atomic number, mass number, electronic configuration, valency, isotopes, and isobars.
Every element in the periodic table is made of atoms. But what is inside an atom? Scientists spent over a century answering that question. Each atomic model changed how students understand chemistry and physics. Chapter 8 of Class 9 Science explains this journey step by step. Important questions class 9 science chapter 8 test atomic models, subatomic particles, electronic configuration, isotopes and isobars. This article covers every CBSE 2026 exam format, from diagram-based questions to case studies.
Key Takeaways
| Chapter |
Class 9 Science Chapter 8 |
| Topic |
Journey Inside the Atom |
| Syllabus |
CBSE 2026 |
| Question Types |
VSA, Short Answer, Long Answer, Diagram-Based, Table-Based, Assertion-Reason, Case Study |
| Key Topics |
Subatomic Particles, Thomson’s Model, Rutherford’s Model, Bohr’s Model, Atomic Number, Mass Number, Electronic Configuration, Valency, Isotopes, Isobars |
Class 9 Science Chapter 8 Journey Inside the Atom: Topics Covered
Every topic below has appeared in CBSE school exams. Identify your weak areas here before starting the question bank.
- Structure of atom: electron, proton, neutron
- Thomson's atomic model: plum pudding
- Rutherford's atomic model: nuclear model
- Bohr's atomic model: energy levels
- Modern atomic model
- Atomic number and mass number
- Number of protons, neutrons, and electrons
- Electronic configuration: Bohr shell model
- Valence electrons and valency
- Isotopes and their applications
- Isobars and how they differ from isotopes
Important Questions Class 9 Science Chapter 8 with Answers
Atomic structure questions class 9 reward students who understand the historical development of atomic models. Examiners test why each model was replaced and what evidence led to the next.
Every class 9 science chapter 8 question answer set here connects concept to evidence. Work through VSA first, then short answer, then long answer.
Very Short Answer Important Questions Class 9 Science Chapter 8
Q1. What is an atom?
Ans. An atom is the smallest particle of an element that retains the chemical properties of that element.
It has a nucleus containing protons and neutrons. Electrons move around the nucleus in shells.
Q2. Name the three subatomic particles of an atom.
Ans. The three subatomic particles are electron, proton, and neutron.
Q3. What is the charge on an electron, proton, and neutron?
Ans.
| Particle |
Charge |
| Electron |
-1 |
| Proton |
+1 |
| Neutron |
0 |
Q4. What is atomic number?
Ans. Atomic number is the number of protons in the nucleus of an atom.
It is denoted by Z. It uniquely identifies an element.
Q5. What is mass number?
Ans. Mass number is the total number of protons and neutrons in the nucleus.
Mass number = Number of protons + Number of neutrons
Q6. Define valency.
Ans. Valency is the combining capacity of an atom.
For elements with 1 to 4 valence electrons, valency equals the number of valence electrons. For elements with 5 to 8 valence electrons, valency = 8 - valence electrons.
Q7. What are isotopes?
Ans. Isotopes are atoms of the same element with the same atomic number but different mass numbers.
They have the same number of protons but different numbers of neutrons.
Q8. What are isobars?
Ans. Isobars are atoms of different elements with the same mass number but different atomic numbers.
They have different numbers of protons and electrons.
Short Answer Questions Class 9 Science Chapter 8 with Answers
Short answers from this chapter should mention the model, evidence, and limitation wherever needed.
Atomic structure questions class 9 often test why one model replaced another.
Q1. State the postulates of Thomson’s atomic model. Why was it replaced?
Ans. Thomson proposed that an atom is a sphere of positive charge with electrons embedded in it.
The atom was overall neutral because positive and negative charges balanced each other.
This model was replaced after Rutherford’s gold foil experiment. Rutherford showed that positive charge is concentrated in a tiny nucleus, not spread throughout the atom.
Q2. Describe Rutherford’s gold foil experiment and its conclusions.
Ans. Rutherford fired alpha particles at a very thin gold foil and observed their path on a detector screen.
Observations:
- Most alpha particles passed straight through the foil.
- Some alpha particles were deflected.
- Very few alpha particles bounced back.
Conclusions:
- The atom is mostly empty space.
- The nucleus is very small and positively charged.
- Most of the atom’s mass is concentrated in the nucleus.
Q3. What are the limitations of Rutherford’s atomic model?
Ans. Rutherford’s model could not explain why electrons do not lose energy and fall into the nucleus.
According to classical physics, a charged particle moving in a circular path should radiate energy.
If electrons lost energy continuously, atoms would collapse. But atoms are stable. Rutherford’s model also failed to explain atomic spectra.
Q4. What did Bohr propose to overcome Rutherford’s limitations?
Ans. Bohr proposed that electrons move in fixed circular orbits called energy levels or shells.
Electrons do not radiate energy while moving in these fixed orbits.
Energy is absorbed or emitted only when an electron jumps from one orbit to another. This explained atomic stability and hydrogen’s line spectrum.
Q5. How do you find the number of neutrons in an atom?
Ans. Number of neutrons = Mass number - Atomic number
For chlorine-35:
Mass number = 35
Atomic number = 17
Number of neutrons = 35 - 17
= 18
Q6. Write the electronic configuration of sodium with atomic number 11.
Ans. Sodium has 11 electrons.
K shell = 2 electrons
L shell = 8 electrons
M shell = 1 electron
Electronic configuration = 2, 8, 1
Valence electrons = 1
Valency = 1
Long Answer Important Questions Class 9 Science Chapter 8
Long answers should not only define the models. They should explain what each model answered and what it failed to explain.
Use comparison tables whenever two or more models are asked together.
Q1. Compare Thomson’s, Rutherford’s, and Bohr’s atomic models.
Ans.
| Model |
Main Idea |
Correct Contribution |
Limitation |
| Thomson’s model |
Atom is a positive sphere with embedded electrons |
Showed electrons are part of atoms |
Could not explain nucleus or gold foil experiment |
| Rutherford’s model |
Atom has a small positive nucleus and orbiting electrons |
Proved atom is mostly empty space |
Could not explain atomic stability |
| Bohr’s model |
Electrons move in fixed energy levels |
Explained atomic stability and hydrogen spectrum |
Could not fully explain atoms with many electrons |
Thomson’s model was the first to include electrons inside the atom. Rutherford’s model introduced the nucleus. Bohr’s model improved Rutherford’s idea by giving electrons fixed energy levels.
Q2. Explain electronic configuration using Bohr’s shell model. Give three examples.
Ans. In Bohr’s shell model, electrons are arranged in shells around the nucleus.
The shells are filled from inside to outside.
At Class 9 level:
- K shell can hold 2 electrons.
- L shell can hold 8 electrons.
- M shell can hold 8 electrons.
- The outermost shell should not have more than 8 electrons.
Examples:
Carbon has atomic number 6. Its electronic configuration is 2, 4. Valency = 4.
Oxygen has atomic number 8. Its electronic configuration is 2, 6. Valency = 2.
Sodium has atomic number 11. Its electronic configuration is 2, 8, 1. Valency = 1.
Elements with similar outer-shell electrons show similar chemical behaviour.
Diagram and Model-Based Questions Class 9 Science Chapter 8
Diagram-based questions usually test Thomson’s model, Rutherford’s model, Bohr’s model, and electronic shell diagrams.
Draw the nucleus clearly, label charges, and show electron shells neatly.
Thomson Atomic Model Questions Class 9
Q1. Draw Thomson’s atomic model and label its key features.
Ans. Draw a sphere of positive charge with electrons embedded throughout it.
Label these parts:
- Positive sphere
- Electrons embedded in the sphere
- Overall neutral atom
This model is also called the plum pudding model.
Q2. What was the main contribution of Thomson’s model?
Ans. Thomson’s model showed that atoms contain electrons.
This was important because Dalton had considered the atom indivisible. Thomson proved that atoms have smaller particles inside them.
Rutherford Atomic Model Questions Class 9
Rutherford atomic model questions class 9 appear in diagram, short-answer, long-answer, and assertion-reason formats.
The gold foil experiment is the most important part of this section.
Q1. Draw Rutherford’s atomic model and label the nucleus and electron paths.
Ans. Draw a tiny nucleus at the centre of the atom.
Then draw electrons revolving around it.
Label these parts:
- Small dense nucleus
- Positive charge in nucleus
- Electrons around the nucleus
- Empty space inside the atom
Q2. What evidence proved that the nucleus is very small?
Ans. Most alpha particles passed straight through the gold foil.
This showed that most of the atom is empty space.
Only a very small number of particles bounced back. This proved that the nucleus is very small, dense, and positively charged.
Bohr Atomic Model Questions Class 9
Bohr atomic model questions class 9 test energy levels and shell diagrams.
Always fill electrons from the innermost shell first.
Q1. Draw Bohr’s model of carbon with atomic number 6.
Ans. Carbon has 6 electrons.
K shell = 2 electrons
L shell = 4 electrons
Draw the nucleus at the centre. Then draw two shells around it.
Label the nucleus with 6 protons. Show 2 electrons in K shell and 4 electrons in L shell.
Q2. Draw Bohr’s model of oxygen with atomic number 8.
Ans. Oxygen has 8 electrons.
K shell = 2 electrons
L shell = 6 electrons
Draw the nucleus and two shells. Show 2 electrons in the first shell and 6 electrons in the second shell.
Valence electrons = 6
Valency = 2
Modern Atomic Model Questions Class 9
Q1. How does the modern atomic model differ from Bohr’s model?
Ans. Bohr’s model shows electrons moving in fixed circular orbits.
The modern atomic model does not treat electrons as moving in fixed paths.
It describes regions called orbitals where the probability of finding an electron is high. This model works better for atoms with many electrons.
Important Questions on Subatomic Particles Class 9
Subatomic particles questions class 9 test the charge, mass, location, and discovery of electrons, protons, and neutrons.
Remember this order clearly: electrons are outside the nucleus, while protons and neutrons are inside the nucleus.
Electron, Proton and Neutron Questions Class 9
Q1. Compare electrons, protons, and neutrons.
Ans.
| Property |
Electron |
Proton |
Neutron |
| Charge |
-1 |
+1 |
0 |
| Relative mass |
1/1836 |
1 |
1 |
| Location |
Outside nucleus |
Inside nucleus |
Inside nucleus |
| Discovered by |
J.J. Thomson |
Goldstein |
Chadwick |
Q2. Which subatomic particle determines the identity of an element?
Ans. Proton determines the identity of an element.
The number of protons is the atomic number. Every element has a unique atomic number.
Charge and Mass of Subatomic Particles Questions Class 9
Q1. Why is an atom electrically neutral despite containing charged particles?
Ans. An atom has equal numbers of protons and electrons.
Protons are positively charged. Electrons are negatively charged.
The equal and opposite charges cancel each other. So, the atom has no net charge.
Location of Electrons, Protons and Neutrons Questions Class 9
Q1. Where are protons, neutrons, and electrons located in an atom?
Ans. Protons and neutrons are located inside the nucleus.
Electrons move in shells around the nucleus.
The nucleus is tiny and dense. The region outside the nucleus contains mostly empty space.
Atomic Number and Mass Number Questions Class 9
Atomic number and mass number questions class 9 include definitions, calculations, and table-completion questions.
The main formulas are simple: atomic number = protons, and mass number = protons + neutrons.
Atomic Number Questions Class 9 Science
Q1. Define atomic number. How does it differ from mass number?
Ans. Atomic number is the number of protons in the nucleus.
Mass number is the total number of protons and neutrons.
Atomic number identifies the element. Mass number identifies the isotope or nuclear mass of that atom.
Q2. An element has atomic number 17. Name it and state the number of electrons in a neutral atom.
Ans. Atomic number 17 belongs to chlorine.
In a neutral atom, number of electrons = number of protons.
So, chlorine has 17 electrons.
Mass Number Questions Class 9 Science
Q1. Calculate the mass number of an element with 15 protons and 16 neutrons.
Ans. Mass number = Protons + Neutrons
= 15 + 16
= 31
The element is phosphorus-31.
Q2. An element has mass number 23 and atomic number 11. Find the number of neutrons.
Ans. Number of neutrons = Mass number - Atomic number
= 23 - 11
= 12
The element is sodium-23.
Protons, Neutrons and Electrons Table Questions Class 9
Q1. Complete the table below.
Ans.
| Element |
Atomic Number |
Mass Number |
Protons |
Neutrons |
Electrons |
| Hydrogen |
1 |
1 |
1 |
0 |
1 |
| Carbon |
6 |
12 |
6 |
6 |
6 |
| Nitrogen |
7 |
14 |
7 |
7 |
7 |
| Oxygen |
8 |
16 |
8 |
8 |
8 |
| Sodium |
11 |
23 |
11 |
12 |
11 |
| Chlorine |
17 |
35 |
17 |
18 |
17 |
| Calcium |
20 |
40 |
20 |
20 |
20 |
Electronic Configuration Important Questions Class 9
Electronic configuration questions class 9 connect Chapter 8 with the next chemistry chapter.
Students who understand shell filling, valence electrons, and valency can answer bonding questions more easily.
Bohr Shell Configuration Questions Class 9
Q1. Write the electronic configuration of Mg, Al, S, and Ar.
Ans.
| Element |
Atomic Number |
Electronic Configuration |
| Magnesium |
12 |
2, 8, 2 |
| Aluminium |
13 |
2, 8, 3 |
| Sulphur |
16 |
2, 8, 6 |
| Argon |
18 |
2, 8, 8 |
Q2. Why does the second shell hold a maximum of 8 electrons?
Ans. The maximum number of electrons in a shell is given by 2n².
For the second shell, n = 2.
Maximum electrons = 2 × 2²
= 2 × 4
= 8
So, the second shell can hold a maximum of 8 electrons.
Valence Electrons Questions Class 9 Science
Q1. What are valence electrons? Why are they important?
Ans. Valence electrons are electrons present in the outermost shell of an atom.
They determine the chemical behaviour and valency of the element.
Elements with the same number of valence electrons often show similar chemical properties.
Q2. Find the valence electrons for Na, Cl, and Ca.
Ans.
Sodium has configuration 2, 8, 1.
Valence electrons = 1
Chlorine has configuration 2, 8, 7.
Valence electrons = 7
Calcium has configuration 2, 8, 8, 2.
Valence electrons = 2
Valency Questions Class 9 Science
Q1. How do you calculate valency from electronic configuration?
Ans. If valence electrons are 1 to 4, valency equals the number of valence electrons.
If valence electrons are 5 to 8, valency = 8 - valence electrons.
If the outer shell is complete, valency is 0.
Q2. Find the valency of oxygen, nitrogen, neon, and potassium.
Ans.
Oxygen has configuration 2, 6.
Valency = 8 - 6 = 2
Nitrogen has configuration 2, 5.
Valency = 8 - 5 = 3
Neon has configuration 2, 8.
Valency = 0
Potassium has configuration 2, 8, 8, 1.
Valency = 1
Isotopes and Isobars Important Questions Class 9
Isotopes and isobars questions class 9 test one key difference.
Isotopes have the same atomic number. Isobars have the same mass number.
Isotopes Questions Class 9 Science
Q1. What are isotopes? Give two examples with atomic details.
Ans. Isotopes are atoms of the same element with the same atomic number but different mass numbers.
Examples:
| Isotope |
Atomic Number |
Mass Number |
| Protium |
1 |
1 |
| Deuterium |
1 |
2 |
| Tritium |
1 |
3 |
| Chlorine-35 |
17 |
35 |
| Chlorine-37 |
17 |
37 |
All isotopes of the same element have similar chemical properties because they have the same electronic configuration.
Q2. Name two applications of isotopes.
Ans.
- Iodine-131 is used in treatment of thyroid cancer.
- Carbon-14 is used for dating ancient organic materials.
Isobars Questions Class 9 Science
Q1. What are isobars? Give one example.
Ans. Isobars are atoms of different elements with the same mass number but different atomic numbers.
Example:
Calcium-40 has atomic number 20.
Argon-40 has atomic number 18.
Both have mass number 40, but they are different elements.
Q2. Do isobars have similar chemical properties?
Ans. No. Isobars do not have similar chemical properties.
They have different atomic numbers and different electronic configurations.
Chemical properties depend mainly on electronic configuration, not mass number.
Isotopes vs Isobars Questions Class 9
Q1. Distinguish between isotopes and isobars.
Ans.
| Feature |
Isotopes |
Isobars |
| Atomic number |
Same |
Different |
| Mass number |
Different |
Same |
| Number of protons |
Same |
Different |
| Number of neutrons |
Different |
Different |
| Element |
Same |
Different |
| Chemical properties |
Similar |
Different |
| Example |
C-12 and C-14 |
Ca-40 and Ar-40 |
Atomic Models Comparison Questions Class 9
Atomic models comparison questions class 9 test the progression from Dalton to the modern model.
Do not memorise the models separately. Learn which evidence each model could or could not explain.
Thomson vs Rutherford Atomic Model Questions Class 9
Q1. Compare Thomson’s and Rutherford’s atomic models.
Ans.
| Feature |
Thomson’s Model |
Rutherford’s Model |
| Positive charge |
Spread throughout atom |
Concentrated in nucleus |
| Electrons |
Embedded in positive sphere |
Revolve around nucleus |
| Nucleus |
No nucleus |
Small, dense nucleus |
| Empty space |
Not explained |
Atom is mostly empty space |
| Main limitation |
Could not explain gold foil experiment |
Could not explain atomic stability |
Rutherford vs Bohr Atomic Model Questions Class 9
Q1. How did Bohr’s model improve Rutherford’s model?
Ans. Rutherford’s model could not explain why electrons do not fall into the nucleus.
Bohr proposed that electrons revolve in fixed energy levels.
In these fixed orbits, electrons do not radiate energy. This made the atom stable in Bohr’s model.
Dalton to Modern Atomic Model Timeline Questions Class 9
Q1. Arrange atomic models in historical order. State one contribution of each.
Ans.
| Scientist |
Year |
Contribution |
| Dalton |
1808 |
Proposed atomic theory and indivisible atoms |
| Thomson |
1897 |
Discovered electrons and proposed plum pudding model |
| Rutherford |
1911 |
Discovered the nucleus through gold foil experiment |
| Bohr |
1913 |
Proposed fixed energy levels for electrons |
| Modern model |
Later development |
Replaced fixed orbits with probability-based orbitals |
Each model improved upon the limitations of the earlier model.
Class 9 Science Chapter 8 Assertion Reason Questions
Class 9 science chapter 8 assertion reason questions test whether students understand the evidence behind atomic structure.
Read the assertion and reason separately before choosing the answer.
Directions:
(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.
Q1. Assertion (A): Isotopes of the same element have identical chemical properties. Reason (R): Isotopes have the same atomic number and therefore the same electronic configuration and valency.
Ans. (a) Both A and R are true, and R is the correct explanation of A.
Chemical properties depend on electronic configuration. Isotopes of the same element have the same atomic number and electronic configuration.
Q2. Assertion (A): Rutherford’s model could not explain why electrons do not collapse into the nucleus. Reason (R): Classical physics says a charged particle in circular motion continuously radiates energy and would spiral inward.
Ans. (a) Both A and R are true, and R is the correct explanation of A.
Rutherford’s model could not solve this problem. Bohr explained atomic stability through fixed energy levels.
Q3. Assertion (A): Neon is chemically inert. Reason (R): Neon has 8 valence electrons, a completely filled outer shell, and valency zero.
Ans. (a) Both A and R are true, and R is the correct explanation of A.
A filled outer shell makes neon stable. Its valency is zero.
Q4. Assertion (A): Isobars have different chemical properties even though their mass numbers are the same. Reason (R): Isobars have different atomic numbers and therefore different electronic configurations.
Ans. (a) Both A and R are true, and R is the correct explanation of A.
Chemical properties depend on electronic configuration, not mass number.
Q5. Assertion (A): Thomson’s atomic model was incorrect. Reason (R): Rutherford’s gold foil experiment showed the atom is mostly empty space with a tiny nucleus.
Ans. (a) Both A and R are true, and R is the correct explanation of A.
Thomson’s model had no nucleus. Rutherford’s experiment proved that a central nucleus exists.
Case Study Questions Class 9 Science Chapter 8
Case study questions in Chapter 8 usually give atomic number, mass number, neutrons, or isotope information.
Identify all given values before calculating.
Case Study 1: Element X
An element X has mass number 35 and 18 neutrons.
Q1. Find the atomic number of element X.
Ans. Atomic number = Mass number - Number of neutrons
= 35 - 18
= 17
Element X is chlorine.
Q2. Write the electronic configuration of element X.
Ans. Chlorine has atomic number 17.
So, it has 17 electrons in a neutral atom.
Electronic configuration = 2, 8, 7
Valence electrons = 7
Valency = 1
Q3. Element Y has atomic number 17 and mass number 37. What is the relationship between X and Y?
Ans. X and Y are isotopes.
Both have atomic number 17, but different mass numbers.
X has mass number 35. Y has mass number 37.
Case Study 2: Atomic Science Applications
Scientists use different isotopes in medicine, agriculture, and energy production. Iodine-131 treats thyroid cancer. Cobalt-60 sterilises surgical equipment. Carbon-14 dates ancient objects. Uranium-235 powers nuclear reactors.
Each application depends on specific nuclear properties of the isotope used.
Q1. Why can different isotopes of the same element have different nuclear applications?
Ans. Isotopes have different numbers of neutrons.
Different neutron numbers give different nuclear stability and radiation properties.
For example, iodine-131 emits radiation useful for treating thyroid tissue. Carbon-14 decays at a known rate, which helps in dating ancient organic materials.
Q2. Iodine-131 has atomic number 53. Find its number of neutrons.
Ans. Number of neutrons = Mass number - Atomic number
= 131 - 53
= 78
So, iodine-131 has 78 neutrons.
Q3. Two elements have mass number 40. Element A has atomic number 18. Element B has atomic number 20. What is their relationship?
Ans. A and B are isobars.
They have the same mass number but different atomic numbers.
Element A is argon. Element B is calcium. They have different chemical properties.
Important Definitions and Formulas Class 9 Science Chapter 8
| Term |
Definition / Formula |
| Atomic number (Z) |
Number of protons in the nucleus |
| Mass number (A) |
A = Z + N, or protons + neutrons |
| Neutrons |
N = A - Z |
| Electronic configuration |
Distribution of electrons in shells: 2, 8, 18... |
| Valence electrons |
Electrons in the outermost shell |
| Valency |
Valence electrons, if 1-4; or 8 - valence electrons, if 5-8 |
| Isotopes |
Same Z, different A; same element |
| Isobars |
Same A, different Z; different elements |
| Thomson’s model |
Electrons embedded in a sphere of positive charge |
| Rutherford’s model |
Small dense nucleus; electrons orbit outside |
| Bohr’s model |
Electrons in fixed energy levels; emit or absorb energy on jumping |