Science Exploration Notes Class 9 English Medium 2026-27

Science Exploration Chapter 8. Journey Inside the Atom

CBSE Notes New Syllabus: Class 9

Assignments - What Have You Learned?

Chapter 8. Journey Inside the Atom

This assignment is based on the complete chapter and follows the latest CBSE competency-based assessment pattern. It includes objective, descriptive, application-based, case-study and HOTS questions to strengthen conceptual understanding of atomic structure, atomic models, subatomic particles, electronic configuration and valency. 

Chapter Assignment

1. One Word Answer

1. The smallest particle of an element that retains its chemical properties.
2. The ancient Indian philosopher who proposed the concept of Parmanu.
3. The Greek philosopher who used the word "Atomos".
4. The scientist who proposed the first scientific atomic theory.
5. The negatively charged subatomic particle.
6. The positively charged particle present in the nucleus.
7. The neutral particle present inside the nucleus.
8. The scientist who discovered the nucleus through the gold foil experiment.
9. The number of protons present in an atom.
10. The outermost shell of an atom.

2. Fill in the Blanks

1. Matter is made up of tiny particles called __________.
2. The electron was discovered by __________.
3. Rutherford used a thin __________ foil in his experiment.
4. The atomic number is represented by the symbol __________.
5. Mass number is equal to the number of __________ and __________.
6. The maximum number of electrons in a shell is given by __________.
7. The first shell can accommodate a maximum of __________ electrons.
8. The combining capacity of an atom is called __________.
9. Electrons are arranged in different __________ around the nucleus.
10. The outermost shell is known as the __________ shell.

3. True or False

1. Dalton proposed the first scientific atomic theory.
2. Electrons carry a positive charge.
3. Most of the mass of an atom is concentrated in the nucleus.
4. Thomson's model successfully explained the gold foil experiment.
5. Bohr proposed that electrons move in fixed energy levels.
6. Atomic number is equal to the number of neutrons.
7. Neutrons carry no electrical charge.
8. A neutral atom has equal numbers of protons and electrons.
9. The valence shell is the outermost shell of an atom.
10. Helium has a valency of zero.

4. Match the Following

5. Very Short Answer Questions

1. Define an atom.
2. Who proposed the concept of Parmanu?
3. State one postulate of Dalton's atomic theory.
4. Who discovered the electron?
5. What is Thomson's Plum Pudding Model?
6. State one conclusion of Rutherford's gold foil experiment.
7. Define atomic number.
8. Define mass number.
9. What is electronic configuration?
10. What is valency?

6. Short Answer Questions

1. Explain Dalton's atomic theory with its main postulates.
2. Describe the cathode ray experiment and its conclusions.
3. Explain Thomson's atomic model with its merits and limitations.
4. Describe Rutherford's gold foil experiment and the conclusions drawn from it.
5. Explain the main features of Bohr's atomic model.
6. Differentiate between Thomson's and Rutherford's atomic models.
7. Differentiate between atomic number and mass number.
8. Explain the Bohr–Bury rules for electronic configuration.
9. Describe the arrangement of electrons in different shells.
10. Explain the concept of valency with suitable examples.

7. Long Answer Questions

1. Describe the historical development of atomic theory from Acharya Kanada to Niels Bohr.
2. Explain the discoveries of electron, proton and neutron and discuss their importance.
3. Describe Rutherford's gold foil experiment with observations and conclusions.
4. Explain Bohr's atomic model and discuss how it overcame the limitations of Rutherford's model.
5. Explain atomic number, mass number and standard representation of an atom with examples.
6. Describe the Bohr–Bury rules and explain the electronic configuration of elements up to atomic number 20.
7. Explain the concepts of valence shell, valence electrons and valency.
8. Compare Dalton's, Thomson's, Rutherford's and Bohr's atomic models.

8. Case Study Questions

Case Study – 1

A scientist passed electricity through a discharge tube containing gas at low pressure and observed rays travelling from the cathode towards the anode.

1. Name the experiment.
2. Who performed this experiment?
3. Which particle was discovered?
4. What is the charge on this particle?

Case Study – 2

During an experiment, most alpha particles passed straight through a thin gold foil, while a few were deflected.

1. Name the scientist who performed this experiment.
2. What does this observation suggest about the structure of the atom?
3. Which part of the atom contains most of its mass?
4. Which atomic model was proposed after this experiment?

Case Study – 3

A student writes the electronic configuration of sodium as 2, 8, 1.

1. How many electrons are present in sodium?
2. How many valence electrons does it have?
3. What is its valency?
4. Which shell is the valence shell?

Case Study – 4

An atom has 17 protons, 18 neutrons and 17 electrons.

1. Find its atomic number.
2. Find its mass number.
3. Is the atom electrically neutral?
4. Write the electronic configuration of this atom.

Case Study – 5

Two students compare Thomson's and Rutherford's atomic models during a classroom discussion.

1. Which model proposed the existence of the nucleus?
2. Which model is known as the Plum Pudding Model?
3. Why was Thomson's model rejected?
4. Which limitation of Rutherford's model was explained by Bohr?

9. Competency-Based Questions

1. Why was Dalton's atomic theory modified after the discovery of subatomic particles?
2. How did Rutherford's experiment prove that most of the atom is empty space?
3. Why are electrons not found inside the nucleus?
4. How does the atomic number help identify an element?
5. Why is the outermost shell important in chemical reactions?
6. Explain why helium is chemically stable.
7. How does electronic configuration help predict the valency of an element?
8. Why did Bohr introduce fixed energy levels for electrons?
9. Explain why atoms are electrically neutral.
10. How did the discovery of the neutron improve the understanding of atomic structure?

10. HOTS Questions

1. Explain why each new atomic model replaced the previous one instead of completely rejecting it.
2. An atom has atomic number 15 and mass number 31. Calculate the number of protons, neutrons and electrons.
3. Why could Rutherford's model not explain the stability of atoms?
4. How would chemistry be affected if electrons were present inside the nucleus instead of outside it?
5. Compare Thomson's, Rutherford's and Bohr's atomic models based on their ability to explain atomic structure.
6. Why is the atomic number considered more important than the mass number for identifying an element?
7. How does the electronic configuration of an element determine its chemical behaviour?
8. Why are noble gases generally chemically inactive?
9. Explain how the discovery of subatomic particles changed Dalton's idea of indivisible atoms.
10. Draw a flow chart showing the development of atomic theory from ancient ideas to Bohr's atomic model and explain each stage briefly.