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

Reviewed by · Updated on

NCERT Solutions PDF Class 9 PDF

This comprehensive set of NCERT Solutions for CBSE Class 9 Science Exemplar, Chapter 4, focuses on the Structure of the Atom. It covers fundamental concepts like atomic number, mass number, electronic distribution, and the historical development of atomic models, including Rutherford's scattering experiment and Dalton's atomic theory. The solutions provide clear explanations for multiple-choice questions, helping students grasp the properties of subatomic particles and the arrangement of electrons in atoms. These solutions are designed to reinforce learning and aid students in their exam preparation by offering step-by-step guidance and clarifying complex ideas related to atomic structure.

Home

Quick info

BoardCBSE
ClassClass 9
SubjectScience Exemplar
Session2026
LanguageEnglish
TypeNCERT Solutions
ChapterChapter 4

Chapter summary

Chapter 4 of the CBSE Class 9 Science Exemplar, focusing on the Structure of the Atom, delves into the fundamental building blocks of matter. These NCERT Solutions explain key concepts such as atomic number, mass number, isotopes, and isobars. They also cover the historical evolution of atomic models, including Rutherford's alpha-particle scattering experiment and its implications, leading to the discovery of the nucleus. The solutions clarify the electronic configuration of elements and the distribution of electrons in different shells, crucial for understanding chemical behavior.

Learning outcomes

  • Understand the electronic distribution in atoms based on atomic number.
  • Identify the discovery of the atomic nucleus through Rutherford's experiment.
  • Determine the representation of an element using its atomic and mass numbers.
  • Recall the key explanations provided by Dalton's atomic theory.
  • Analyze the validity of statements regarding Rutherford's atomic model.
  • Differentiate between atomic number and mass number in elements.

Topics covered

Paper topics

  • Structure of the Atom
  • Atomic Models
  • Rutherford's Scattering Experiment
  • Nucleus
  • Dalton's Atomic Theory
  • Subatomic Particles
  • Electrons
  • Protons
  • Neutrons
  • Atomic Number
  • Mass Number
  • Electronic Distribution

Important topics

  • Rutherford's alpha-particle scattering experiment and discovery of the nucleus
  • Dalton's atomic theory and its explanations
  • Relationship between atomic number, mass number, protons, neutrons, and electrons
  • Electronic configuration and distribution in shells

PDF preview

Read page by page below. PDF is streamed from the official NCERT website — no download button on this page.

Loading document …
Page of
Loading page …

Questions and Solutions

Question 1

Which of the following correctly represent the electronic distribution in the Mg atom?

3, 8, 1

2, 8, 2

1, 8, 3

8, 2, 2

Solution: The correct electronic distribution for the Magnesium (Mg) atom is 2, 8, 2. Magnesium has an atomic number of 12, meaning it has 12 electrons. These electrons are arranged in shells around the nucleus. The first shell (K) can hold a maximum of 2 electrons. The second shell (L) can hold a maximum of 8 electrons. The remaining 2 electrons are placed in the third shell (M). Therefore, the electronic configuration is 2, 8, 2.

Question 2

Rutherford's 'alpha ($\alpha$) particles scattering experiment' resulted in the discovery of which part of the atom?

Electron

Proton

Nucleus in the atom

Atomic mass

Solution: Rutherford's 'alpha ($\alpha$) particles scattering experiment' resulted in the discovery of the **Nucleus in the atom**. During the experiment, it was observed that while most alpha particles passed straight through the gold foil, a few were deflected, and some even bounced back. This indicated the presence of a small, dense, positively charged region within the atom, which Rutherford named the nucleus.

Question 3

An element X has 15 electrons and 16 neutrons. Which of the following is the correct representation of the element?

^{31}_{15}X

^{31}_{16}X

^{16}_{15}X

^{15}_{16}X

Solution: The correct representation of the element is ^{31}_{15}X. In a neutral atom, the number of electrons is equal to the number of protons. Since element X has 15 electrons, it has 15 protons. The atomic number (Z) is equal to the number of protons, so the atomic number is 15. The mass number (A) is the sum of protons and neutrons. In this case, the mass number is 15 (protons) + 16 (neutrons) = 31. The representation of an element is typically shown as ^{A}_{Z}X, where A is the mass number and Z is the atomic number.

Question 4

Dalton's atomic theory successfully explained which of the following laws?

(i) Law of conservation of mass

(ii) Law of constant composition

(iii) Law of radioactivity

(iv) Law of multiple proportion

Options:

(i), (ii) and (iii)

(i), (iii) and (iv)

(ii), (iii) and (iv)

(i), (ii) and (iv)

Solution: Dalton's atomic theory successfully explained the **(i) Law of conservation of mass**, **(ii) Law of constant composition**, and **(iv) Law of multiple proportion**. The theory stated that atoms are indivisible and cannot be created or destroyed in a chemical reaction (explaining the law of conservation of mass), and that atoms of a given element combine in simple whole-number ratios to form compounds (explaining the law of constant composition and multiple proportions). The theory did not provide explanations for radioactivity.

Question 5

Which of the following statements about Rutherford's model of atom are correct?

(i) It considered the nucleus as positively charged.

(ii) It established that the $\alpha$-particles are four times as heavy as a hydrogen atom.

(iii) It can be compared to the solar system.

(iv) It was in agreement with Thomson's model.

Options:

(i) and (iii)

(ii) and (iii)

(i) and (iv)

only (i)

Solution: The correct statements about Rutherford's model of the atom are **(i) It considered the nucleus as positively charged** and **(iii) It can be compared to the solar system**. The deflection of positively charged alpha particles by the atom's central part indicated that this part, the nucleus, was positively charged. Rutherford also likened the atom to the solar system, where electrons orbit the nucleus much like planets orbit the sun. Statement (ii) is incorrect as alpha particles are helium nuclei, significantly heavier than hydrogen atoms. Statement (iv) is incorrect because Rutherford's model fundamentally differed from Thomson's model.

Question 6

Which of the following statements are true for an element?

(i) Atomic number = number of protons + number of electrons

(ii) Mass number = number of protons + number of neutrons

(iii) Atomic mass = number of protons = number of neutrons

(iv) Atomic number = number of protons = number of electrons

Options:

(i) and (ii)

(i) and (iii)

(ii) and (iii)

(ii) and (iv)

Solution: The true statements for an element are **(ii) Mass number = number of protons + number of neutrons** and **(iv) Atomic number = number of protons = number of electrons**. The atomic number (Z) defines an element and is equal to the number of protons in the nucleus. In a neutral atom, the number of electrons is also equal to the number of protons. The mass number (A) is the sum of protons and neutrons in the nucleus. Statement (i) is incorrect because the atomic number is not the sum of protons and electrons. Statement (iii) is incorrect because atomic mass is approximately equal to the mass number, but not necessarily equal to the number of protons and neutrons individually, and it's the sum that matters for mass number.

Common mistakes

  • Confusing atomic number with mass number.
  • Incorrectly calculating the number of protons, neutrons, or electrons.
  • Misinterpreting the conclusions of Rutherford's scattering experiment.
  • Errors in representing an element with its atomic and mass numbers.

Revision tips

  • Review the electronic configuration rules for different elements.
  • Understand the significance of Rutherford's alpha-particle scattering experiment.
  • Practice representing elements using their atomic and mass numbers.
  • Memorize the key postulates of Dalton's atomic theory and their explanations.
  • Focus on the differences and similarities between various atomic models.

Practice MCQs

Q1. Which of the following correctly represents the electronic distribution in the Magnesium (Mg) atom, given its atomic number is 12?

Q2. Rutherford's 'alpha ($\alpha$) particles scattering experiment' led to which significant discovery about the atom?

Q3. An element X has 15 electrons and 16 neutrons. Which of the following is the correct representation of this element?

Q4. Which of the following fundamental laws was successfully explained by Dalton's atomic theory?

Q5. Which of the following statements about Rutherford's model of the atom are considered correct?

Q6. For any given element, which of the following statements are true regarding its atomic structure?

Frequently asked questions

What is the main focus of Chapter 4, 'Structure of the Atom', in the Class 9 Science Exemplar?

Chapter 4 focuses on the fundamental structure of atoms, including the discovery and properties of subatomic particles (protons, neutrons, electrons), historical atomic models like Rutherford's, and the concepts of atomic number and mass number.

How does Rutherford's alpha-particle scattering experiment help us understand atomic structure?

This experiment led to the discovery of the atomic nucleus. It showed that the atom has a small, dense, positively charged center (the nucleus) where most of the mass is concentrated, and that electrons orbit this nucleus.

What is the difference between atomic number and mass number?

The atomic number represents the number of protons in an atom's nucleus (and also the number of electrons in a neutral atom), defining the element. The mass number is the total number of protons and neutrons in the nucleus.

How are electrons distributed in an atom according to the concepts in this chapter?

Electrons are distributed in specific energy shells or orbits around the nucleus. The distribution follows rules where the inner shells are filled first, with the first shell holding a maximum of 2 electrons, the second up to 8, and so on.

What are the key takeaways from Dalton's atomic theory mentioned in these solutions?

Dalton's theory successfully explained the laws of conservation of mass and constant composition. It proposed that atoms are indivisible and that atoms of the same element are identical.

How can these NCERT Solutions help in exam preparation?

These solutions provide clear, step-by-step explanations for complex concepts and questions, helping students understand the underlying principles of atomic structure and revise effectively for their exams.

Important links

Content reviewed by the NCERT Help team. Editorial Team and update policy

NCERT Solutions PDF PDF on NCERT Help. URL unchanged for search indexing.