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

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NCERT Solutions PDF Class 9 PDF

This chapter delves into the fundamental structure of the atom, exploring the contributions of various scientists like Thomson and Rutherford. The NCERT Solutions for Class 9 Science, Chapter 4, provide clear explanations of atomic models, including Thomson's 'plum pudding' model and Rutherford's nuclear model. It details the discovery and properties of subatomic particles such as electrons, protons, and neutrons, and explains why atoms are electrically neutral. The solutions also touch upon Bohr's model, illustrating the arrangement of electrons in different shells. These solutions are designed to help students grasp complex concepts related to atomic structure, aiding in their preparation for exams by offering step-by-step explanations and reinforcing key principles of atomic theory.

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Quick info

BoardCBSE
ClassClass 9
SubjectScience
Session2026
LanguageEnglish
TypeNCERT Solutions
ChapterChapter 4

Chapter summary

Chapter 4, Structure of the Atom, focuses on the historical development of atomic models. It covers Thomson's model, Rutherford's nuclear model, and Bohr's model, explaining the existence and properties of subatomic particles like electrons and protons. The solutions clarify the concept of atomic neutrality and the arrangement of electrons in shells, providing a foundational understanding of atomic composition.

Learning outcomes

  • Understand the concept of canal rays and their composition.
  • Explain why an atom is electrically neutral.
  • Describe Thomson's model of the atom and its explanation for neutrality.
  • Identify the subatomic particle present in the nucleus according to Rutherford's model.
  • Visualize Bohr's model of the atom with electrons in different shells.

Topics covered

Paper topics

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

Important topics

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

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Questions and Solutions

Question 1

What are canal rays?
Solution: Canal rays, also known as Goldstein rays, are positively charged radiations. These rays were discovered by Eugen Goldstein in 1886. They are composed of positively charged particles, which are now identified as protons.

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 is a negatively charged particle, and a proton is a positively charged particle. The magnitude of the charge on an electron is equal to the magnitude of the charge on a proton. Therefore, when one of each is present, their charges cancel each other out, resulting in a 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 model of the atom, an atom is visualized as a positively charged sphere 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 model of the atom, the subatomic particle present in the nucleus of an atom is the proton. Rutherford's experiments led him to conclude that the atom's center, the nucleus, contains these positively charged particles.

Question 3

Draw a sketch of Bohr's model of an atom with three shells.
Solution: Bohr's model of an atom depicts electrons orbiting the nucleus in specific energy levels or shells. A sketch showing three shells (K, L, and M) would illustrate these orbits around a central, positively charged nucleus. The innermost shell is the K shell, followed by the L shell, and then the M shell. Electrons occupy these shells at distinct energy levels.

Bohr's model of atom with three shells

Common mistakes

  • Confusing the charges of subatomic particles.
  • Misunderstanding the basis of atomic neutrality in different models.
  • Difficulty in sketching or interpreting atomic models.

Revision tips

  • Review the key postulates of Thomson's and Rutherford's atomic models.
  • Focus on understanding the charge balance that makes an atom neutral.
  • Practice sketching Bohr's model to visualize electron shells.
  • Ensure you can define and differentiate between electrons and protons.

Practice MCQs

Q1. What are canal rays composed of?

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

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

Q4. What charge does an electron carry?

Q5. If an atom has one proton and one electron, what is its net charge?

Frequently asked questions

What are canal rays?

Canal rays are positively charged radiations discovered by E. Goldstein. They consist of positively charged particles, which are now known as protons.

Why is an atom electrically neutral?

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

What did Thomson's model suggest about atomic structure?

Thomson's model proposed that an atom is a sphere of positive charge with negatively charged electrons embedded within it, similar to plums in a pudding. The total positive and negative charges are equal, making the atom neutral.

According to Rutherford's model, what is located in the atom's nucleus?

Rutherford's model states that the nucleus of an atom contains the positively charged particles, which are protons.

How does Bohr's model represent electrons?

Bohr's model depicts electrons revolving around the nucleus in specific, fixed orbits or shells, each associated with a definite energy level.

What is the charge of an electron and a proton?

An electron carries a negative charge, while a proton carries a positive charge. The magnitude of their charges is equal.

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