Complete Solutions and Summary of Structure of the Atom – NCERT Class 9, Science, Chapter 4 – Summary, Questions, Answers, Extra Questions

Detailed summary and explanation of Chapter 4 “Structure of the Atom” with all question answers, extra questions, and solutions from NCERT Class IX, Science.

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Categories: NCERT, Class IX, Science, Summary, Extra Questions, Structure of the Atom, Chapter 4

Tags: Structure of the Atom, Class 9 Science, NCERT, Chapter 4, Subatomic Particles, Thomson’s Model, Rutherford’s Model, Bohr’s Model, Neutrons, Atomic Number, Mass Number, Valency, Isotopes, Isobars, Summary, Answers, Extra Questions

Structure of the Atom - Complete Study Guide

Chapter Overview

1897 Electron Discovery

1911 Rutherford's Model

1913 Bohr's Model

1932 Neutron Discovery

What You'll Learn

Sub-atomic Particles

Understanding electrons, protons, and neutrons as building blocks of atoms.

Atomic Models

Exploring Thomson's, Rutherford's, and Bohr's models of atomic structure.

Electron Distribution

Learning how electrons are arranged in shells and their role in valency.

Atomic Number & Mass

Defining atomic number, mass number, isotopes, and isobars.

Historical Context

This chapter explores the evolution of atomic theory from Dalton's indivisible atoms to the discovery of sub-atomic particles. Key experiments like Thomson's electron identification, Goldstein's canal rays, Rutherford's alpha-scattering, and Chadwick's neutron discovery shaped our understanding. Bohr's model addressed stability issues, leading to concepts of electron shells, valency, isotopes, and isobars.

Key Highlights

Atoms consist of protons, neutrons in the nucleus, and electrons in orbits. Atomic number equals protons, mass number is protons plus neutrons. Isotopes have same atomic number but different mass numbers, while isobars have same mass but different atomic numbers.

Comprehensive Chapter Summary

1. Charged Particles in Matter

Atoms are divisible and contain charged particles. J.J. Thomson discovered the electron (e⁻) in 1897, a negatively charged particle with negligible mass. E. Goldstein found canal rays in 1886, leading to the proton (p⁺), positively charged with mass 2000 times the electron. Activities like combing dry hair or rubbing glass rod with silk show charging by friction, indicating atoms have charged parts.

2. The Structure of an Atom

Thomson’s Model

Atom as a positively charged sphere with embedded electrons, like a Christmas pudding or watermelon, electrically neutral. J.J. Thomson (1856-1940), British physicist, Nobel in 1906, directed Cavendish Lab.

Rutherford’s Model

From alpha-particle scattering: Atom has a small, dense, positive nucleus; electrons revolve around it. Most space empty. E. Rutherford (1871-1937), 'Father' of nuclear physics, Nobel in 1908. Drawback: orbiting electrons unstable.

Bohr’s Model

Electrons in discrete orbits without radiating energy; overcomes Rutherford's instability. Neils Bohr (1885-1962), Nobel in 1922, professor at Copenhagen. Orbits K,L,M,N or n=1,2,3...

3. Neutrons

Discovery

J. Chadwick discovered neutrons (n) in 1932: neutral particles with mass nearly equal to protons, present in nucleus except hydrogen. Mass of atom = protons + neutrons.

4. Electron Distribution in Orbits (Shells)

Bohr-Bury Scheme

Maximum electrons per shell: 2n² (K=2, L=8, M=18, etc.). Outermost shell max 8; inner shells filled first. Schematic for first 18 elements in Fig. 4.4. Activity: Make static model of first 18 elements' configuration.

5. Valency

Combining Capacity

Valence electrons in outermost shell determine valency: lose/gain/share to achieve octet. E.g., H/Li/Na valency 1, Mg 2, Al 3, F 1 (gains 1), O 2. Inert elements like He (2), Ne/Ar (8) valency 0.

Inert Elements

Elements with full octet (e.g., He=2, Ne=8) have valency 0, chemically inert. Atoms react to attain octet.

6. Atomic Number and Mass Number

Atomic number (Z): number of protons/electrons, defines element. Mass number (A): protons + neutrons. Notation: ⁿₐX. E.g., N as ¹⁴₇N. Mass due to nucleons (p+n).

7. Isotopes and Isobars

Isotopes: same Z, different A (e.g., H¹, H²=D, H³=T). Similar chemical, different physical properties. Average mass calculated from abundance, e.g., Cl 35.5u. Applications: U fuel, Co cancer, I goitre. Isobars: same A, different Z (e.g., Ca⁴⁰, Ar⁴⁰).

Questions and Answers from Chapter

Short Questions

Q1. What are canal rays?

Answer: Positively charged radiations discovered by E. Goldstein.

Q2. If an atom contains one electron and one proton, will it carry any charge or not?

Answer: No charge, as charges balance.

Q3. Which sub-atomic particle is present in the nucleus of an atom?

Answer: Proton.

Q4. Name the three sub-atomic particles of an atom.

Answer: Electron, proton, neutron.

Q5. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

Answer: 2 neutrons.

Q6. What is the atomic number of the atom if number of electrons is 8 and number of protons is 8?

Answer: 8.

Q7. What is the charge on the atom if number of electrons is 8 and number of protons is 8?

Answer: Neutral.

Q8. What are the limitations of J.J. Thomson’s model of the atom?

Answer: Could not explain results of other experiments.

Q9. What are the limitations of Rutherford’s model of the atom?

Answer: Electron orbit instability.

Q10. What is atomic number?

Answer: Number of protons.

Q11. What is mass number?

Answer: Protons + neutrons.

Q12. What are isotopes?

Answer: Same Z, different A.

Q13. What are isobars?

Answer: Same A, different Z.

Q14. What is valency?

Answer: Combining capacity.

Q15. Name the discoverer of electron.

Answer: J.J. Thomson.

Medium Questions

Q1. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.

Answer: Positive sphere balances embedded negative electrons, making atom neutral. (3 marks)

Q2. On the basis of Rutherford’s model of an atom, which sub-atomic particle is present in the nucleus?

Answer: Positively charged protons occupy the nucleus. (3 marks)

Q3. What do you think would be the observation if the α-particle scattering experiment is carried out using a foil of a metal other than gold?

Answer: Similar deflections but fewer rebounds due to different density. (3 marks)

Q4. Write the distribution of electrons in carbon and sodium atoms.

Answer: Carbon: 2,4; Sodium: 2,8,1. (3 marks)

Q5. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?

Answer: 10 electrons (2+8). (3 marks)

Q6. How will you find the valency of chlorine, sulphur and magnesium?

Answer: Chlorine: 1 (gains 1); Sulphur: 2 (gains 2); Magnesium: 2 (loses 2). (3 marks)

Q7. With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.

Answer: Oxygen: 16 u; Sulphur: 32 u. (3 marks)

Q8. For the symbol H, D and T tabulate three sub-atomic particles found in each of them.

Answer: H: 1p,0n,1e; D:1p,1n,1e; T:1p,2n,1e. (3 marks)

Q9. Write the electronic configuration of any one pair of isotopes and isobars.

Answer: Isotopes (C-12, C-14): both 2,4; Isobars (Ca-40, Ar-40): Ca 2,8,8,2; Ar 2,8,8. (3 marks)

Q10. What are the limitations of J.J. Thomson’s model of the atom?

Answer: Failed to explain results from Rutherford and others. (3 marks)

Q11. What are the limitations of Rutherford’s model of the atom?

Answer: Electron would lose energy and collapse into nucleus. (3 marks)

Q12. Define valency by taking examples of silicon and oxygen.

Answer: Silicon: 4 (2,8,4); Oxygen: 2 (2,6, gains 2). (3 marks)

Q13. Give any two uses of isotopes.

Answer: Uranium as nuclear fuel; Cobalt for cancer treatment. (3 marks)

Q14. Na+ has completely filled K and L shells. Explain.

Answer: Na loses 1 electron, leaving 2,8 configuration. (3 marks)

Q15. Composition of the nuclei of two atomic species X and Y are given as under. Give the mass numbers of X and Y.

Answer: X:12; Y:14. (3 marks)

Q16. Draw a sketch of Bohr’s model of an atom with three shells.

Answer: Nucleus in center, three concentric circles for K, L, M shells with electrons. (3 marks)

Q17. What is the charge on the atom? (From Q1 on page 6)

Answer: Neutral if electrons = protons. (3 marks)

Q18. On the basis of Thomson's model, explain neutrality.

Answer: Positive and negative charges equal. (3 marks)

Q19. Explain Rutherford's observations.

Answer: Most alpha passed straight, some deflected small, few large/rebound. (3 marks)

Q20. How are electrons distributed in shells?

Answer: 2n² formula, stepwise filling. (3 marks)

Long Questions

Q1. Compare the properties of electrons, protons and neutrons.

Answer: Electrons: negative charge, negligible mass, orbit nucleus. Protons: positive charge, mass 1 u, in nucleus. Neutrons: neutral, mass 1 u, in nucleus. Electrons balance protons for neutrality; neutrons contribute to mass. All are sub-atomic; electrons discovered by Thomson, protons by Goldstein, neutrons by Chadwick.

Q2. Describe Bohr’s model of the atom.

Answer: Bohr proposed electrons revolve in discrete orbits without radiating energy. Orbits are energy levels (K,L,M,N). Overcomes Rutherford's drawback by ensuring stability. Electrons jump orbits absorbing/emitting energy. Basis for electron distribution rules.

Q3. Compare all the proposed models of an atom given in this chapter.

Answer: Thomson: Positive sphere with embedded electrons, neutral but couldn't explain scattering. Rutherford: Small positive nucleus, orbiting electrons, empty space; unstable orbits. Bohr: Discrete stable orbits, no radiation; explains stability. All build on sub-atomic particles; Bohr most successful for basic structure.

Q4. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Answer: Max electrons/shell: 2n² (n=1,2,...). Outermost max 8. Fill inner shells first. E.g., K=2, L=8, M=18. Shown in Fig. 4.4 for elements 1-18.

Q5. Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and (iv) Isobars.

Answer: (i) Atomic number: Protons, e.g., C=6. (ii) Mass number: Protons+neutrons, e.g., C=12. (iii) Isotopes: Same Z different A, e.g., C-12, C-14. (iv) Isobars: Same A different Z, e.g., Ca-40, Ar-40.

Q6. If bromine atom is available in the form of, say, two isotopes ⁷⁹₃₅Br (49.7%) and ⁸¹₃₅Br (50.3%), calculate the average atomic mass of bromine atom.

Answer: (79*0.497) + (81*0.503) = 80 u. Weighted average based on abundance.

Q7. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes ¹⁶₈X and ¹⁸₈X in the sample?

Answer: Let y% be ¹⁸X, then (100-y)% ¹⁶X. 16.2 = [16*(100-y) + 18*y]/100 → y=10%, (100-y)=90%.

Q8. If Z = 3, what would be the valency of the element? Also, name the element.

Answer: Lithium (Li), configuration 2,1; valency 1 (loses 1 electron).

Q9. Composition of the nuclei of two atomic species X and Y are given as under. What is the relation between the two species?

Answer: Isotopes (same Z=6, different A=12,14).

Q10. For the following statements, write T for True and F for False. (a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.

Answer: F (Thomson proposed positive sphere with electrons).

Q11. For the following statements, write T for True and F for False. (b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.

Answer: F (Neutron is fundamental particle).

Q12. Rutherford’s alpha-particle scattering experiment was responsible for the discovery of

Answer: (a) Atomic Nucleus.

Q13. Isotopes of an element have

Answer: (c) different number of neutrons.

Q14. Number of valence electrons in Cl⁻ ion are:

Answer: (b) 8.

Q15. Which one of the following is a correct electronic configuration of sodium?

Answer: (d) 2,8,1.

Q16. Explain the alpha particle scattering experiment led by Rutherford.

Answer: Alpha particles fired at gold foil; most passed straight, some deflected, few rebounded. Concluded nucleus small, dense, positive; atom mostly empty.

Q17. What are the drawbacks of Rutherford's model?

Answer: Orbiting electrons accelerate, radiate energy, spiral into nucleus; atom unstable, contrary to observation.

Q18. Describe the Bohr-Bury scheme for electron distribution.

Answer: Max 2n² electrons/shell, outermost 8 max, fill stepwise. Examples from table 4.1.

Q19. How is valency determined for elements close to octet?

Answer: Subtract from 8; e.g., F (7 electrons) valency 1, gains 1.

Q20. Explain average atomic mass with chlorine example.

Answer: Cl isotopes 35 (75%), 37 (25%); average (35*0.75 + 37*0.25) = 35.5 u.

Q21. Complete the following table (from exercise 19).

Answer: Fill based on PDF table: e.g., 9p 10n 9e F; 16p 16n 16e S, etc.

Q22. What is the relation between X and Y in exercise 13?

Answer: Isotopes, same protons 6, different neutrons 6 and 8.

Q23. For T/F (c) The mass of an electron is about 1/2000 times that of proton.

Answer: T.

Q24. For T/F (d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.