Chapter 4: Structure of the Atom Case Study Questions | science Class 9 CBSE

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Chapter 4: Structure of the Atom Case Study Questions | science Class 9 CBSE | ByteNotes.in

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In a classroom experiment, a student rubbed a comb on dry hair and noticed it attracted small bits of paper. Later, the teacher explained that this phenomenon indicates the presence of charged particles in matter.

Question 1: Q1. What does this activity demonstrate about matter?

Rubbing transfers electrons between objects.
This shows that atoms contain charged particles.
Matter can develop static electricity by losing or gaining charges.

Question 2: Q2. Which particles are involved in such charging?

Electrons are transferred during rubbing.
Protons remain in the nucleus and do not move.
Only electrons participate in static charge development.

Question 3: Q3. What does this suggest about the structure of atoms?

Atoms are divisible into subatomic particles.
Presence of negatively charged electrons is confirmed.
Supports the idea that atoms are not indivisible, unlike Dalton's view.

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J.J. Thomson proposed a model in which electrons were embedded in a sphere of positive charge, much like seeds in a watermelon.

Question 1: Q1. What are the key features of Thomson’s model?

Atom has a uniformly distributed positive sphere.
Electrons are embedded within this sphere.
Overall atom remains electrically neutral.

Question 2: Q2. What was the major limitation of this model?

Could not explain results of scattering experiments.
Failed to describe actual placement of electrons.
Didn’t explain the existence of nucleus or atomic stability.

Question 3: Q3. How was the atom visualised in this model?

Similar to a pudding with embedded dry fruits.
Positive charge spread throughout; electrons as points inside.
No distinct nucleus was suggested.

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In Rutherford’s experiment, alpha particles were directed at a thin gold foil. Most passed through, some deflected, and a few bounced back.

Question 1: Q1. What were Rutherford’s main observations?

Most alpha particles passed straight through the foil.
Some were deflected at small angles.
A few rebounded indicating dense central mass.

Question 2: Q2. What did these observations reveal about atomic structure?

Atom is mostly empty space.
Positive charge and mass concentrated in a tiny nucleus.
Electrons revolve around this central nucleus.

Question 3: Q3. What was the major drawback of Rutherford’s model?

Could not explain electron stability.
Revolving electrons should lose energy and spiral into nucleus.
Didn’t account for atomic stability observed in nature.

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Bohr proposed that electrons revolve in fixed energy levels and do not radiate energy while in these orbits, addressing Rutherford’s limitations.

Question 1: Q1. What are the key postulates of Bohr’s atomic model?

Electrons revolve in discrete, fixed orbits (shells).
Each orbit has a fixed energy level.
Electrons do not lose energy while revolving in these orbits.

Question 2: Q2. How did Bohr’s model explain atomic stability?

Electrons do not emit radiation in stable orbits.
They remain in fixed shells unless energy is gained or lost.
Prevents collapse into the nucleus.

Question 3: Q3. How are these shells represented?

Shells are denoted as K, L, M, N or n=1,2,3,4...
Each shell can hold a specific number of electrons.
Electron distribution follows Bohr-Bury rules.

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J. Chadwick discovered the neutron — a neutral particle in the nucleus, nearly equal in mass to the proton.

Question 1: Q1. What is a neutron and how is it represented?

Neutron is a subatomic particle with no charge.
It is represented by 'n'.
Mass is nearly equal to that of a proton.

Question 2: Q2. Where is the neutron found in an atom?

Neutrons are present in the nucleus.
They combine with protons to form nucleons.
Absent in hydrogen atom (most common isotope).

Question 3: Q3. What is the role of neutrons in the atom?

Contribute to atomic mass.
Help stabilize the nucleus by reducing repulsion between protons.
Do not affect the charge of the atom.

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Chapter sections

Chapter 4: Structure of the Atom | Case Study Questions

Passage

Question 1: Q1. What does this activity demonstrate about matter?

Question 2: Q2. Which particles are involved in such charging?

Question 3: Q3. What does this suggest about the structure of atoms?

Passage

Question 1: Q1. What are the key features of Thomson’s model?

Question 2: Q2. What was the major limitation of this model?

Question 3: Q3. How was the atom visualised in this model?

Passage

Question 1: Q1. What were Rutherford’s main observations?

Question 2: Q2. What did these observations reveal about atomic structure?

Question 3: Q3. What was the major drawback of Rutherford’s model?

Passage

Question 1: Q1. What are the key postulates of Bohr’s atomic model?

Question 2: Q2. How did Bohr’s model explain atomic stability?

Question 3: Q3. How are these shells represented?

Passage

Question 1: Q1. What is a neutron and how is it represented?

Question 2: Q2. Where is the neutron found in an atom?

Question 3: Q3. What is the role of neutrons in the atom?

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