CBSE Class 9 Science Chapter 4: Structure of Atom NCERT Solutions

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NCERT Solutions PDF Class 9 PDF

This resource provides comprehensive NCERT Solutions for Class 9 Science, Chapter 4: Structure of Atom. It covers fundamental concepts related to atomic structure, including the discovery of subatomic particles like electrons and protons, and the evolution of atomic models. Solutions explain Thomson's model, Rutherford's model, and Bohr's model of the atom, detailing how each contributed to our understanding of atomic neutrality and the arrangement of particles. The questions address the nature of canal rays, the charge of an atom with equal numbers of protons and electrons, and the components of the atomic nucleus. These solutions are designed to help students grasp the key principles of atomic structure, clarify doubts, and prepare effectively for their examinations by offering step-by-step explanations and clear reasoning.

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Quick info

BoardCBSE
ClassClass 9
SubjectScience
Session2026
LanguageEnglish
TypeNCERT Solutions
ChapterChapter 4: Structure of Atom

Chapter summary

Chapter 4 of the Class 9 Science NCERT Solutions focuses on the Structure of the Atom. It delves into the historical development of atomic models, starting from early theories to more refined ones like Thomson's, Rutherford's, and Bohr's models. The solutions explain the discovery and properties of subatomic particles (electrons, protons) and their role in atomic structure and neutrality. Key concepts like canal rays and the composition of the nucleus are also addressed, providing a foundational understanding of atomic physics.

Learning outcomes

  • Understand the nature and discovery of canal rays.
  • Explain the concept of atomic neutrality based on subatomic particles.
  • Describe Thomson's model of the atom and its explanation for neutrality.
  • Identify the subatomic particle located in the nucleus according to Rutherford's model.
  • Visualize Bohr's model of the atom with distinct electron shells.

Topics covered

Paper topics

  • Canal Rays
  • Protons
  • Atomic Neutrality
  • Thomson's Atomic Model
  • Rutherford's Atomic Model
  • Nucleus
  • Bohr's Atomic Model
  • Electron Shells

Important topics

  • Atomic Models (Thomson, Rutherford, Bohr)
  • Subatomic Particles (Protons, Electrons)
  • Atomic Neutrality
  • Structure of the Nucleus

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Questions and Solutions

Question 1

What are canal rays?
Solution: Canal rays are streams of positively charged radiations that were discovered by Eugen Goldstein in 1886. These rays are composed of positively charged particles, which are known as protons. They travel from the negative electrode towards the positive electrode in a discharge tube.

Question 2

If an atom contains one electron and one proton, will it carry any charge or not?
Solution: An atom containing one electron and one proton will not carry any net charge. This is because an electron possesses a negative charge, while a proton possesses a positive charge of equal magnitude. When these two particles are present in an atom, their charges cancel each other out, resulting in an electrically neutral atom.

Question 1

On the basis of Thomson's model of an atom, explain how the atom is neutral as a whole.
Solution: According to J.J. Thomson's atomic model, an atom is visualized as a sphere of positive charge in which negatively charged electrons are embedded. Thomson proposed that the magnitude of the positive charge distributed throughout the sphere is exactly equal to the total magnitude of the negative charges of the electrons. This balance of equal and opposite charges makes the atom electrically neutral as a whole.

Question 2

On the basis of Rutherford's model of an atom, which subatomic particle is present in the nucleus of an atom?
Solution: Based on Ernest Rutherford's atomic model, the subatomic particle present in the nucleus of an atom is the proton. Rutherford's experiments led him to conclude that the atom's positive charge and most of its mass are concentrated in a small, dense central region called the nucleus.

Question 3

Draw a sketch of Bohr's model of an atom with three shells.
Solution: Bohr's model of the atom depicts electrons orbiting the nucleus in specific, discrete energy levels or shells. For an atom with three shells, the sketch would show a central, positively charged nucleus, surrounded by three concentric orbits representing the electron shells. These shells are typically labeled K, L, and M, starting from the innermost shell closest to the nucleus. The K shell is the first shell, the L shell is the second, and the M shell is the third shell. Electrons occupy these shells according to specific rules.

Bohr's model of an atom with three shells showing nucleus and K, L, M shells.

Common mistakes

  • Confusing the charge of subatomic particles.
  • Misunderstanding the concept of atomic neutrality.
  • Incomplete recall of the components of different atomic models.

Revision tips

  • Review the properties of electrons and protons to understand atomic charge.
  • Compare and contrast the key features of Thomson's, Rutherford's, and Bohr's atomic models.
  • Practice sketching atomic models, especially Bohr's model with multiple shells.
  • Focus on the reasoning behind atomic neutrality in each model.

Practice MCQs

Q1. What are canal rays primarily composed of?

Q2. According to Thomson's model, why is an atom electrically neutral?

Q3. Which subatomic particle is found in the nucleus of an atom according to Rutherford's model?

Q4. An atom with one electron and one proton is:

Q5. In Bohr's model, which shell is closest to the nucleus?

Frequently asked questions

What are canal rays and who discovered them?

Canal rays are streams of positively charged particles, consisting mainly of protons. They were discovered by Eugen Goldstein in 1886.

Why is an atom considered electrically neutral?

An atom is neutral because it contains an equal number of positively charged protons and negatively charged electrons. These opposite charges cancel each other out.

What did Rutherford's model state about the nucleus?

Rutherford's model proposed that the nucleus of an atom is positively charged and contains protons. It is located at the center of the atom.

How does Thomson's model explain atomic neutrality?

Thomson's model suggests that an atom is a sphere of positive charge with negatively charged electrons embedded in it. The total positive and negative charges are equal, resulting in neutrality.

What are the shells in Bohr's model of the atom?

Bohr's model describes electrons orbiting the nucleus in specific energy levels or shells, denoted as K, L, M, N, etc., with the K shell being the closest to the nucleus.

What is the significance of having one electron and one proton in an atom?

An atom with one electron (negative charge) and one proton (positive charge) will be electrically neutral because the magnitudes of their charges are equal and opposite.

Important links

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