CBSE Class 11 Chemistry Exemplar Chapter 2: Structure of Atom NCERT Solutions

Reviewed by · Updated on

NCERT Solutions PDF Class 11 PDF

This comprehensive set of NCERT Solutions for CBSE Class 11 Chemistry Exemplar, Chapter 2, focuses on the Structure of the Atom. It delves into fundamental concepts like Rutherford's scattering experiment, Bohr's atomic model, and the properties of subatomic particles. The solutions provide clear explanations for multiple-choice questions covering electron configurations, probability density plots of orbitals (1s and 2s), characteristics of cathode rays, and the properties of electrons. It also addresses the limitations of the Thomson model of the atom. These solutions are designed to help students grasp the core principles of atomic structure, understand experimental evidence, and prepare effectively for their examinations by clarifying complex topics and providing step-by-step reasoning.

Home

Quick info

BoardCBSE
ClassClass 11
SubjectChemistry Exemplar
Session2026
LanguageEnglish
TypeNCERT Solutions
ChapterChapter 2

Chapter summary

This chapter's NCERT Solutions for Class 11 Chemistry Exemplar, Chapter 2, "Structure of Atom," cover essential topics including the outcomes of Rutherford's alpha-particle scattering experiment, Bohr's model postulates, and the nature of subatomic particles. It clarifies concepts related to electron configurations, the probability distribution of electrons in s-orbitals, and the characteristics of cathode rays. The solutions also highlight the limitations of early atomic models like Thomson's. This resource is crucial for understanding the historical development and modern understanding of atomic structure.

Learning outcomes

  • Understand the conclusions that can and cannot be derived from Rutherford's alpha-particle scattering experiment.
  • Identify correct and incorrect ground state electronic configurations based on quantum numbers and orbital filling rules.
  • Interpret probability density plots for s-orbitals and understand the concept of radial probability distribution.
  • Recall and differentiate the characteristics of cathode rays and their properties.
  • Compare the mass of an electron with other subatomic particles like neutrons.
  • Evaluate the applicability and limitations of the Thomson model of the atom in explaining atomic properties.

Topics covered

Paper topics

  • Rutherford's Alpha-Particle Scattering Experiment
  • Thomson's Model of the Atom
  • Bohr's Model of the Atom
  • Subatomic Particles (Electrons, Protons, Neutrons)
  • Cathode Rays
  • Electronic Configuration
  • Quantum Numbers (implied)
  • Atomic Orbitals (s-orbitals)
  • Probability Density
  • Radial Probability Distribution
  • Stability of Atoms
  • Atomic Radius vs. Nuclear Radius

Important topics

  • Conclusions from Rutherford's Scattering Experiment
  • Electronic Configuration Rules and Exceptions
  • Properties of Cathode Rays
  • Limitations of Thomson's and Rutherford's Models
  • Understanding s-orbital shapes and probability distributions

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 conclusions could not be derived from Rutherford's $\alpha$-particle scattering experiment?
  1. Most of the space in the atom is empty
  2. The radius of the atom is about $10^{-10}$ m while that of nucleus is $10^{-15}$ m
  3. Electrons move in a circular path of fixed energy called orbits
  4. Electrons and the nucleus are held together by electrostatic forces of attraction
Solution: The correct answer is (c). Rutherford's $\alpha$-particle scattering experiment led to the conclusion that the atom is mostly empty space (1), and that there is a small, dense, positively charged nucleus at the center. The experiment also provided estimates for the relative sizes of the atom and the nucleus (2). The electrostatic attraction between the positively charged nucleus and the negatively charged electrons holds the atom together (4). However, the concept that electrons move in specific circular paths of fixed energy, known as orbits or stationary states, was proposed later by Niels Bohr as part of his atomic model to explain atomic stability and spectra. Rutherford's model did not incorporate this idea.

Question 2

Which of the following options does not represent a ground state electronic configuration of an atom?

(a) $1s^2 2s^2 2p^6 3s^2 3p^6 3d^8 4s^2$ (b) $1s^2 2s^2 2p^6 3s^2 3p^6 3d^9 4s^2$ (c) $1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^1$ (d) $1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^1$

Solution: The incorrect ground state electronic configuration is (b) $1s^2 2s^2 2p^6 3s^2 3p^6 3d^9 4s^2$. According to the Aufbau principle, Hund's rule, and the Pauli exclusion principle, electrons fill orbitals in order of increasing energy. However, atoms tend to achieve greater stability when their subshells are completely filled or half-filled. For an atom with 29 electrons (like Copper), the expected configuration based on filling order would be $1s^2 2s^2 2p^6 3s^2 3p^6 3d^9 4s^2$. But, due to the extra stability associated with a completely filled d-subshell ($3d^{10}$), one electron from the 4s orbital is promoted to the 3d orbital, resulting in the actual ground state configuration of $1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^1$. Configuration (a) represents Nickel (atomic number 28), (c) represents Copper (atomic number 29), and (d) represents Chromium (atomic number 24) or Manganese (atomic number 25) with a half-filled d-subshell, which are stable configurations.

Question 3

The probability density plots of 1s and 2s orbitals are given in the figure. On the basis of the above diagram, which of the following statements is incorrect?

(a) 1s and 2s orbitals are spherical in shape

(b) The probability of finding the electron is maximum near the nucleus

(c) The probability of finding the electron at a given distance is equal in all directions

(d) The probability density of electrons for 2s orbital decreases uniformly as distance from the nucleus increases

Solution: The incorrect statement is (d). The probability density, represented by the density of dots in the figure, indicates the likelihood of finding an electron in a particular region of space. For s-orbitals (like 1s and 2s), the probability density is spherically symmetrical, meaning it is the same in all directions at a given distance from the nucleus (c). Statement (a) is correct as s-orbitals are spherical. Statement (b) is generally true for the 1s orbital, where the probability density is highest at the nucleus, although the radial probability distribution peaks at a certain distance. However, for the 2s orbital, the probability density does not decrease uniformly. It starts at zero at the nucleus, increases to a maximum at a certain distance, then decreases, passes through zero at a radial node, and then increases again to a smaller maximum before decreasing again as the distance from the nucleus increases. Therefore, statement (d) is incorrect.

Question 4

Which of the following statements about the characteristics of cathode rays is not correct?

(a) They start from the cathode and move towards the anode

(b) They travel in straight lines in the absence of an external electrical or magnetic field

(c) Characteristics of cathode rays do not depend upon the material of electrodes in the cathode ray tube

(d) Characteristics of cathode rays depend upon the nature of gas present in the cathode ray tube

Solution: The incorrect statement is (d). Cathode rays are streams of negatively charged particles, now known as electrons, discovered by William Crookes. Their fundamental characteristics, such as their charge-to-mass ratio, are independent of the material of the electrodes (c) and the nature of the gas present in the cathode ray tube. They always originate from the cathode and travel towards the anode (a) in straight lines in the absence of external fields (b).

Question 5

Which of the following statements about the electron is incorrect?

(a) It is a negatively charged particle

(b) The mass of electron is equal to the mass of neutron

(c) It is a basic constituent of all atoms

(d) It is a constituent of cathode rays

Solution: The incorrect statement is (b). An electron is indeed a negatively charged particle (a), a fundamental constituent of all atoms (c), and forms the basis of cathode rays (d). However, the mass of an electron is significantly smaller than the mass of a neutron. The approximate mass of an electron is $9.1 \times 10^{-31}$ kg, while the approximate mass of a neutron is $1.67 \times 10^{-27}$ kg. Thus, their masses are not equal.

Question 6

Which of the following properties of atom could be explained correctly by Thomson model of atom?

(a) Overall neutrality of atom

(b) Spectra of hydrogen atom

(c) Position of electrons, protons and neutrons in atom

(d) Stability of atom

Solution: The property that could be correctly explained by Thomson's model of the atom is (a) the overall neutrality of the atom. Thomson proposed that an atom consists of a sphere of uniformly distributed positive charge, with negatively charged electrons embedded within it. This arrangement, often visualized as 'plums in a pudding', naturally accounted for the fact that atoms are electrically neutral, as the total positive charge balanced the total negative charge of the electrons. However, Thomson's model failed to explain the results of Rutherford's scattering experiment, the stability of atoms, and the discrete line spectra observed for elements like hydrogen.

Common mistakes

  • Confusing conclusions from Rutherford's experiment with Bohr's model postulates.
  • Incorrectly applying Aufbau principle, Hund's rule, or Pauli exclusion principle when determining electronic configurations.
  • Misinterpreting probability density versus radial probability distribution for orbitals.
  • Confusing the properties of cathode rays with anode rays or other subatomic particles.
  • Assuming the mass of an electron is comparable to that of a neutron.

Revision tips

  • Review the key differences between Rutherford's and Bohr's atomic models.
  • Practice writing electronic configurations for various elements, paying attention to exceptions.
  • Visualize the shapes and probability distributions of s-orbitals using the provided diagrams.
  • Memorize the fundamental properties of electrons and cathode rays.
  • Understand why early atomic models like Thomson's were eventually superseded.

Practice MCQs

Q1. Which of the following conclusions could NOT be derived from Rutherford's alpha-particle scattering experiment?

Q2. Which of the following options does NOT represent a valid ground state electronic configuration of an atom?

Q3. Consider the probability density plots for 1s and 2s orbitals. Which statement about these plots is incorrect?

Q4. Which of the following statements is NOT correct regarding the characteristics of cathode rays?

Q5. Which of the following statements about the electron is incorrect?

Q6. Which property of the atom could be correctly explained by Thomson's model of the atom?

Frequently asked questions

What are the main conclusions from Rutherford's alpha-particle scattering experiment?

Rutherford's experiment concluded that the atom is mostly empty space, has a small, dense, positively charged nucleus, and that the electrons orbit this nucleus. However, it could not explain the stability of the atom or the discrete atomic spectra.

Why is the electronic configuration $1s^2 2s^2 2p^6 3s^2 3p^6 3d^9 4s^2$ considered incorrect for a ground state?

This configuration is incorrect because atoms tend towards greater stability. A completely filled d-subshell ($3d^{10}$) is more stable than a nearly filled one ($3d^9$). Therefore, for elements like Copper (atomic number 29), an electron moves from the 4s orbital to the 3d orbital, resulting in the configuration $1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^1$.

What is the difference between probability density and radial probability distribution?

Probability density ($|\Psi|^2$) is the probability of finding an electron per unit volume at a specific point. Radial probability distribution is the probability of finding an electron within a thin spherical shell at a certain distance from the nucleus. For s-orbitals, the probability density is maximum at the nucleus, but the radial probability distribution is zero at the nucleus and peaks at a specific radius.

Are the characteristics of cathode rays dependent on the gas in the tube?

No, the fundamental characteristics of cathode rays (like their charge-to-mass ratio) are independent of the nature of the gas present in the discharge tube or the material of the electrodes. They always consist of electrons.

What did Thomson's model successfully explain?

Thomson's 'plum pudding' model successfully explained the overall electrical neutrality of the atom by proposing that electrons were embedded in a positively charged sphere.

How do these NCERT solutions help in exam preparation?

These solutions provide clear, step-by-step explanations for complex concepts in atomic structure. By rewriting the solutions and expanding on the reasoning, they help students understand the 'why' behind the answers, identify common mistakes, and reinforce their learning for 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.