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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Structure of the Atom
Chapter 4: Science
Complete Study Guide with Interactive Learning
Chapter Overview
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).
Key Concepts and Definitions
Electron
Negatively charged sub-atomic particle, mass ~1/2000 u.
Proton
Positively charged, mass 1 u, in nucleus.
Neutron
Neutral, mass 1 u, in nucleus.
Atomic Number (Z)
Number of protons in nucleus.
Mass Number (A)
Total protons + neutrons.
Valency
Combining capacity based on valence electrons.
Important Facts and Figures
Questions and Answers from Chapter
Short Questions
Q1. What are canal rays?
Q2. If an atom contains one electron and one proton, will it carry any charge or not?
Q3. Which sub-atomic particle is present in the nucleus of an atom?
Q4. Name the three sub-atomic particles of an atom.
Q5. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?
Q6. What is the atomic number of the atom if number of electrons is 8 and number of protons is 8?
Q7. What is the charge on the atom if number of electrons is 8 and number of protons is 8?
Q8. What are the limitations of J.J. Thomson’s model of the atom?
Q9. What are the limitations of Rutherford’s model of the atom?
Q10. What is atomic number?
Q11. What is mass number?
Q12. What are isotopes?
Q13. What are isobars?
Q14. What is valency?
Q15. Name the discoverer of electron.
Medium Questions
Q1. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
Q2. On the basis of Rutherford’s model of an atom, which sub-atomic particle is present in the nucleus?
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?
Q4. Write the distribution of electrons in carbon and sodium atoms.
Q5. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Q6. How will you find the valency of chlorine, sulphur and magnesium?
Q7. With the help of Table 4.1, find out the mass number of oxygen and sulphur atom.
Q8. For the symbol H, D and T tabulate three sub-atomic particles found in each of them.
Q9. Write the electronic configuration of any one pair of isotopes and isobars.
Q10. What are the limitations of J.J. Thomson’s model of the atom?
Q11. What are the limitations of Rutherford’s model of the atom?
Q12. Define valency by taking examples of silicon and oxygen.
Q13. Give any two uses of isotopes.
Q14. Na+ has completely filled K and L shells. Explain.
Q15. Composition of the nuclei of two atomic species X and Y are given as under. Give the mass numbers of X and Y.
Q16. Draw a sketch of Bohr’s model of an atom with three shells.
Q17. What is the charge on the atom? (From Q1 on page 6)
Q18. On the basis of Thomson's model, explain neutrality.
Q19. Explain Rutherford's observations.
Q20. How are electrons distributed in shells?
Long Questions
Q1. Compare the properties of electrons, protons and neutrons.
Q2. Describe Bohr’s model of the atom.
Q3. Compare all the proposed models of an atom given in this chapter.
Q4. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.
Q5. Explain with examples (i) Atomic number, (ii) Mass number, (iii) Isotopes and (iv) Isobars.
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.
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?
Q8. If Z = 3, what would be the valency of the element? Also, name the element.
Q9. Composition of the nuclei of two atomic species X and Y are given as under. What is the relation between the two species?
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.
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.
Q12. Rutherford’s alpha-particle scattering experiment was responsible for the discovery of
Q13. Isotopes of an element have
Q14. Number of valence electrons in Cl⁻ ion are:
Q15. Which one of the following is a correct electronic configuration of sodium?
Q16. Explain the alpha particle scattering experiment led by Rutherford.
Q17. What are the drawbacks of Rutherford's model?
Q18. Describe the Bohr-Bury scheme for electron distribution.
Q19. How is valency determined for elements close to octet?
Q20. Explain average atomic mass with chlorine example.
Q21. Complete the following table (from exercise 19).
Q22. What is the relation between X and Y in exercise 13?
Q23. For T/F (c) The mass of an electron is about 1/2000 times that of proton.
Q24. For T/F (d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
Q25. Explain how atoms achieve octet.
Interactive Knowledge Quiz
Test your understanding of Structure of the Atom
Quick Revision Notes
Sub-atomic Particles
- e⁻: -1, negligible mass
- p⁺: +1, 1 u
- n: 0, 1 u
Atomic Models
- Thomson: Pudding
- Rutherford: Nuclear
- Bohr: Orbits
Electron Shells
- 2n² rule
- Max outer: 8
- Fill inner first
Numbers
- Z: Protons
- A: P+N
- Isotopes: Same Z
Exam Strategy Tips
- Draw models
- Calculate valency
- Compare particles
- Use table 4.1
- Explain experiments
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