Chapter: 8 – Journey Inside the Atom
Maximum Marks: 50
Suggested Time: 1 Hour 30 Minutes

This Class 9 Science Exploration Chapter 8 Sample Paper 1 is designed to help students practise important concepts from “Journey Inside the Atom.” The paper covers subatomic particles, electrons, protons and neutrons, atomic models, Thomson’s model, Rutherford’s model, Bohr’s model, atomic nucleus, atomic number, mass number, and electronic arrangement. Complete solutions are provided at the end.

Sample Paper 1

General Instructions

1. All questions are compulsory.
2. Read each question carefully.
3. Write answers using appropriate scientific terminology.
4. Show all necessary steps in numerical questions.
5. Draw neat and labelled diagrams wherever required.
6. Use the information given in the question wherever applicable.

Section A — Objective Questions

10 × 1 = 10 Marks

Q1. Which subatomic particle carries a negative charge?

a) Proton
b) Neutron
c) Electron
d) Nucleus

Q2. The positively charged particle present in an atom is:

a) Electron
b) Proton
c) Neutron
d) Photon

Q3. Which particle has no electrical charge?

a) Electron
b) Proton
c) Neutron
d) Ion

Q4. According to Rutherford’s model, most of the atom is:

a) Filled with solid matter
b) Empty space
c) Filled with neutrons
d) Filled with electrons only

Q5. The central part of an atom containing protons and neutrons is called:

a) Shell
b) Orbit
c) Nucleus
d) Electron cloud

Q6. Who proposed the nuclear model of the atom based on the alpha-particle scattering experiment?

a) J.J. Thomson
b) Ernest Rutherford
c) Niels Bohr
d) John Dalton

Q7. According to Bohr’s model, electrons revolve around the nucleus in:

a) Random paths
b) Fixed energy levels or shells
c) The nucleus
d) Straight lines only

Q8. The atomic number of an element is equal to the number of:

a) Neutrons
b) Protons
c) Protons + neutrons
d) Electrons + neutrons

Q9. The mass number of an atom is the total number of:

a) Electrons and protons
b) Protons and neutrons
c) Electrons and neutrons
d) Only neutrons

Q10. Which experiment provided evidence for the presence of a small, dense nucleus?

a) Cathode ray experiment
b) Oil-drop experiment
c) Alpha-particle scattering experiment
d) Heating experiment

Section B — Very Short Answer Questions

5 × 2 = 10 Marks

Q11. Name the three main subatomic particles of an atom and state their charges.

Q12. What did Thomson’s model of the atom propose?

Q13. State any two observations from Rutherford’s alpha-particle scattering experiment.

Q14. Why did Rutherford conclude that most of the atom is empty space?

Q15. What is meant by an electron shell or energy level?

Section C — Short Answer Questions

4 × 3 = 12 Marks

Q16. Describe the basic structure of an atom according to the modern understanding.

Q17. Explain two important limitations of Rutherford’s atomic model that led to the development of Bohr’s model.

Q18. An atom contains 11 protons, 12 neutrons and 11 electrons.

Calculate:

a) Its atomic number
b) Its mass number
c) State whether the atom is electrically neutral.

Q19. Compare Thomson’s model and Rutherford’s model of the atom.

Section D — Application-Based Questions

2 × 4 = 8 Marks

Q20. A student observes the following data for an atom:

• Number of protons = 17
• Number of neutrons = 18
• Number of electrons = 17

Answer the following:

a) What is the atomic number?
b) What is the mass number?
c) Is the atom electrically neutral?
d) How many electrons are present outside the nucleus?

Q21. In an experiment similar to Rutherford’s alpha-particle scattering experiment, most alpha particles pass straight through a thin sheet of material, while a very small number are deflected through large angles.

Answer the following:

a) What does the passage of most alpha particles suggest?
b) What does the large-angle deflection of a few particles suggest?
c) What does this tell us about the size of the nucleus compared with the atom?
d) Why was this experiment important in the development of atomic theory?

Section E — Long Answer and Numerical Questions

2 × 5 = 10 Marks

Q22. Describe Rutherford’s alpha-particle scattering experiment. Explain its major observations and the conclusions drawn about the structure of the atom.

Q23. An atom has 19 protons, 20 neutrons and 19 electrons.

Answer the following:

a) Find its atomic number.
b) Find its mass number.
c) Explain why the atom is electrically neutral.
d) Write its electronic arrangement in shells.
e) State the number of electrons in its outermost shell.

SOLUTIONS

Section A — Answers

Q1. c) Electron

Q2. b) Proton

Q3. c) Neutron

Q4. b) Empty space

Q5. c) Nucleus

Q6. b) Ernest Rutherford

Q7. b) Fixed energy levels or shells

Q8. b) Protons

Q9. b) Protons and neutrons

Q10. c) Alpha-particle scattering experiment

Section B — Solutions

Q11. Subatomic Particles

The three main subatomic particles are:

Electrons are found outside the nucleus, while protons and neutrons are located mainly inside the nucleus.

Q12. Thomson’s Atomic Model

J.J. Thomson proposed that an atom was a uniform sphere of positive charge with negatively charged electrons embedded in it.

The model is often described using the analogy of electrons embedded in a positively charged sphere.

According to this model, the positive and negative charges balance each other, making the atom electrically neutral.

Q13. Rutherford’s Experiment — Observations

Two important observations were:

1. Most alpha particles passed straight through the gold foil without significant deflection.
2. A very small number of alpha particles were deflected through large angles, and a few were even reflected backward.

These observations showed that the atom does not have its positive charge spread uniformly throughout its volume.

Q14. Atom Is Mostly Empty Space

In Rutherford’s experiment, most alpha particles passed through the gold foil without being deflected.

This suggested that there was very little matter or concentrated positive charge in most of the atom.

Therefore, Rutherford concluded that most of the volume of an atom is empty space.

Q15. Electron Shell

An electron shell or energy level is a region around the nucleus associated with a particular energy of electrons.

In the simplified Bohr model, electrons occupy specific shells around the nucleus.

These shells are commonly represented as: K, L, M, N

from the innermost to the outermost shell.

Section C — Solutions

Q16. Structure of an Atom

An atom consists of a very small, dense nucleus at its centre.

The nucleus contains:

• Protons, which have positive charge.
• Neutrons, which have no electrical charge.

Electrons, which carry negative charge, occupy regions around the nucleus.

In the simplified shell model, electrons are arranged in different energy levels or shells.

The number of protons determines the atomic number of an element.

Q17. Limitations of Rutherford’s Model

Rutherford’s model successfully explained the presence of a small nucleus but could not explain some important features.

1. Stability of the Atom

According to classical physics, a charged electron moving around the nucleus should continuously lose energy and eventually spiral into the nucleus.

However, atoms are generally stable.

2. Arrangement of Electrons

Rutherford’s model did not specify how electrons are arranged around the nucleus or why electrons should occupy particular energy levels.

These limitations led to the development of Bohr’s model, which introduced specific energy levels for electrons.

Q18. Atomic Number and Mass Number

Given:

Number of protons: 11

Number of neutrons: 12

Number of electrons: 11

a) Atomic Number

Atomic number = number of protons. Z=11​

b) Mass Number

A=Number of protons+Number of neutrons A=11+12 A=23​

c) Electrical Nature

Number of protons = 11

Number of electrons = 11

The positive and negative charges balance each other.

Therefore, the atom is: Electrically neutral​

Q19. Thomson’s Model vs Rutherford’s Model

Rutherford’s model therefore represented a major improvement over Thomson’s model.

Section D — Solutions

Q20. Structure of the Given Atom

Given:

Protons = 17

Neutrons = 18

Electrons = 17

a) Atomic Number

Z=Number of protons Z=17​

b) Mass Number

A=Protons+Neutrons A=17+18 A=35​

c) Electrical Nature

Number of protons = 17

Number of electrons = 17

Therefore, positive and negative charges balance. The atom is electrically neutral​

d) Electrons Outside the Nucleus

Electrons are located outside the nucleus.

Therefore: 17 electrons​

Q21. Alpha-Particle Scattering

a) Most alpha particles pass straight through

This suggests that most of the atom is empty space.

b) Large-angle deflection of a few particles

This suggests that a small, dense, positively charged region exists inside the atom.

c) Size of nucleus

The nucleus is extremely small compared with the overall size of the atom.

d) Importance of the experiment

The experiment provided evidence that:

• the atom contains a small, dense nucleus;
• most of the atom is empty space;
• positive charge and most of the mass are concentrated in the nucleus.

It helped replace Thomson’s model with Rutherford’s nuclear model.

Section E — Solutions

Q22. Rutherford’s Alpha-Particle Scattering Experiment

Rutherford and his collaborators directed a beam of alpha particles towards a very thin sheet of gold foil. A screen around the foil was used to detect where the alpha particles travelled after interacting with the foil.

Major Observations

1. Most alpha particles passed straight through the foil.

This indicated that most of the atom is empty space.

2. Some alpha particles were deflected through small angles.

This showed that the positive charge inside the atom was not uniformly distributed.

3. A very small number of alpha particles were deflected through very large angles or returned backwards.

This suggested that the atom contains a very small, dense and positively charged region capable of strongly repelling the positively charged alpha particles.

Conclusions

Rutherford proposed that:

• The atom contains a tiny, dense nucleus at its centre.
• The nucleus contains positive charge and most of the atom’s mass.
• Most of the atom is empty space.
• Electrons are present around the nucleus.

The experiment was a major step towards our modern understanding of atomic structure.

Q23. Atomic Structure

Given:

Number of protons: 19

Number of neutrons: 20

Number of electrons: 19

a) Atomic Number

Z=Number of protons Z=19​

b) Mass Number

A=Protons+Neutrons A=19+20 A=39​

c) Why is the atom neutral?

Each proton has a charge of +1, while each electron has a charge of −1.

Here: 19 protons=+19 19 electrons=−19

Therefore, the total charge is: +19−19=0

Hence, the atom is: Electrically neutral​

d) Electronic Arrangement

The atom has 19 electrons.

Using the shell arrangement:

• K shell = 2
• L shell = 8
• M shell = 8
• N shell = 1

Therefore: 2, 8, 8, 1​

e) Outermost Electrons

The outermost shell contains: 1 electron​

Final Answers