Class 9 Notes

Class 9 Science Notes Chapter 4 (Chapter 4) – Examplar Problem (English) Book

Philoid

Philoid

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Examplar Problem (English)
Alright class, let's focus on Chapter 4: Structure of the Atom from your NCERT Exemplar. This is a fundamental chapter, and understanding these concepts is crucial not just for your Class 9 exams but also forms the basis for chemistry in higher classes and various government exams. Pay close attention.

Chapter 4: Structure of the Atom - Detailed Notes for Exam Preparation

1. Charged Particles in Matter & Discovery of Subatomic Particles

  • Background: Atoms were initially thought to be indivisible (Dalton's Atomic Theory). However, experiments involving static electricity and electrolysis suggested the presence of charged particles within atoms.
  • Discovery of Electron (e⁻):
    • Discoverer: J.J. Thomson (credited).
    • Experiment: Cathode Ray Tube experiment. Observations showed negatively charged particles moving from cathode to anode.
    • Properties: Negatively charged (-1 unit charge), mass considered negligible (approx. 1/1840th the mass of a hydrogen atom). Electrons are located outside the nucleus.
  • Discovery of Proton (p⁺):
    • Discoverer: E. Goldstein (discovered canal rays/anode rays); Rutherford later identified them as protons.
    • Experiment: Modified Cathode Ray Tube (using perforated cathode). Observed positively charged rays (canal rays).
    • Properties: Positively charged (+1 unit charge), mass taken as 1 atomic mass unit (amu). Protons reside inside the nucleus.
  • Discovery of Neutron (n⁰):
    • Discoverer: James Chadwick (1932).
    • Experiment: Bombardment of light elements (like Beryllium) with alpha particles.
    • Properties: No charge (neutral), mass approximately equal to that of a proton (1 amu). Neutrons also reside inside the nucleus.
  • Conclusion: Atoms are divisible and contain three fundamental subatomic particles: electrons, protons, and neutrons.

2. Atomic Models

  • a) Thomson's Model (Plum Pudding or Watermelon Model - ~1904):

    • Postulates:
      • An atom consists of a positively charged sphere.
      • Electrons (negatively charged) are embedded within this sphere, like plums in a pudding or seeds in a watermelon.
      • The total positive charge equals the total negative charge, making the atom electrically neutral.
    • Limitation: Failed to explain the results of Rutherford's alpha-scattering experiment. It couldn't explain the position of the nucleus and the empty space within the atom.

  • b) Rutherford's Nuclear Model (Based on Alpha-Particle Scattering Experiment - ~1911):

    • Experiment: Fast-moving alpha particles (Helium nuclei, He²⁺) were bombarded onto a very thin gold foil.
    • Observations:
      • Most alpha particles passed straight through the foil undeflected.
      • A small fraction of alpha particles were deflected by small angles.
      • Very few (about 1 in 12,000) alpha particles rebounded (deflected by 180°).
    • Conclusions:
      • Most of the space inside an atom is empty (as most particles passed straight).
      • There is a small, dense, positively charged centre in the atom called the nucleus (which deflected/rebounded the positively charged alpha particles).
      • The nucleus contains almost the entire mass of the atom.
      • Electrons revolve around the nucleus in circular paths (orbits).
      • The size of the nucleus is very small compared to the size of the atom.
    • Drawbacks: Could not explain the stability of the atom. According to classical physics, an accelerating charged particle (like an electron orbiting the nucleus) should radiate energy continuously, eventually spiral into the nucleus, and the atom should collapse. This doesn't happen. It also didn't explain the discrete line spectra observed for elements.

  • c) Bohr's Model of Atom (~1913):

    • Postulates (to overcome Rutherford's limitations):
      • Electrons revolve around the nucleus only in certain special orbits called discrete orbits or energy levels/shells.
      • While revolving in these discrete orbits, electrons do not radiate energy.
      • These orbits or shells are represented by the letters K, L, M, N... or the numbers n = 1, 2, 3, 4...
      • Energy is emitted or absorbed only when an electron jumps from one energy level to another.
    • Significance: Successfully explained the stability of the atom and the line spectra of hydrogen.

3. Distribution of Electrons in Different Shells (Bohr-Bury Scheme)

  • The arrangement of electrons in various energy shells is governed by certain rules:
    • Rule 1: The maximum number of electrons that can be accommodated in a shell is given by the formula 2n², where 'n' is the orbit number or energy level index (n = 1 for K shell, n = 2 for L shell, etc.).
      • K shell (n=1): Max electrons = 2(1)² = 2
      • L shell (n=2): Max electrons = 2(2)² = 8
      • M shell (n=3): Max electrons = 2(3)² = 18
      • N shell (n=4): Max electrons = 2(4)² = 32
    • Rule 2: The maximum number of electrons that can be accommodated in the outermost orbit is 8 (Octet Rule). (Exception: If it's the K shell, the maximum is 2 - Duplet Rule).
    • Rule 3: Electrons are not accommodated in a given shell unless the inner shells are filled. Shells are filled in a step-wise manner.

4. Valency

  • Definition: The combining capacity of an atom of an element. It determines how an atom will react with other atoms.
  • Valence Electrons: Electrons present in the outermost shell of an atom.
  • Determining Valency:
    • If the number of valence electrons is 1, 2, 3, or 4, the valency is equal to the number of valence electrons.
    • If the number of valence electrons is 5, 6, 7, or 8, the valency is usually calculated as (8 - number of valence electrons). (Except for Helium which has 2 valence electrons but valency 0).
    • Atoms with a completely filled outermost shell (like noble gases) have zero valency, meaning they are chemically inert.

5. Atomic Number (Z)

  • Definition: The number of protons present in the nucleus of an atom.
  • Significance: It is the identity of an element. All atoms of the same element have the same atomic number.
  • In a neutral atom: Number of protons (Z) = Number of electrons.
  • Representation: Usually written as a subscript before the element symbol (e.g., ₆C).

6. Mass Number (A)

  • Definition: The total number of protons and neutrons present in the nucleus of an atom. (Protons + Neutrons = Nucleons).
  • Formula: Mass Number (A) = Number of Protons (Z) + Number of Neutrons (n)
  • Therefore: Number of Neutrons (n) = Mass Number (A) - Atomic Number (Z)
  • Representation: Usually written as a superscript before the element symbol (e.g., ¹²C).
  • Complete Notation: ᴬ<0xE2><0x82><0x9 Z>X (e.g., ¹²₆C represents Carbon with Z=6, A=12).

7. Isotopes

  • Definition: Atoms of the same element having the same atomic number (Z) but different mass numbers (A).
  • Reason for difference: They have the same number of protons but a different number of neutrons.
  • Properties: Isotopes of an element have identical chemical properties (determined by electrons/atomic number) but slightly different physical properties (like mass, density).
  • Examples:
    • Hydrogen: ¹₁H (Protium), ²₁H (Deuterium, D), ³₁H (Tritium, T)
    • Carbon: ¹²₆C, ¹³₆C, ¹⁴₆C
    • Chlorine: ³⁵₁₇Cl, ³⁷₁₇Cl (Exists in nature in a ratio of 3:1, leading to fractional atomic mass of 35.5 u)
  • Applications of Isotopes:
    • Uranium isotope (U-235): Used as fuel in nuclear reactors.
    • Cobalt isotope (Co-60): Used in the treatment of cancer (radiotherapy).
    • Iodine isotope (I-131): Used in the treatment of goitre.
    • Carbon isotope (C-14): Used in carbon dating (determining the age of fossils/archaeological samples).

8. Isobars

  • Definition: Atoms of different elements having different atomic numbers (Z) but the same mass number (A).
  • Reason: The total number of nucleons (protons + neutrons) is the same.
  • Properties: Isobars have different chemical properties (as they are different elements) and different physical properties (except for mass number).
  • Examples:
    • ⁴⁰₁₈Ar (Argon) and ⁴⁰₂₀Ca (Calcium). Both have A=40, but Argon has Z=18 (18p, 22n) and Calcium has Z=20 (20p, 20n).

Summary of Atom Structure:

  • The atom consists of a central, dense, positively charged nucleus (containing protons and neutrons).
  • Electrons revolve around the nucleus in specific energy levels or shells.
  • The atom is electrically neutral as the number of protons equals the number of electrons.
  • Atomic Number (Z) = Number of Protons
  • Mass Number (A) = Number of Protons + Number of Neutrons

Multiple Choice Questions (MCQs)

Here are 10 MCQs to test your understanding of this chapter:

  1. Rutherford's alpha-particle scattering experiment was responsible for the discovery of:
    a) Electron
    b) Proton
    c) Atomic Nucleus
    d) Neutron

  2. Which of the following correctly represents the electronic distribution in a Magnesium atom (Atomic number = 12)?
    a) 2, 10
    b) 8, 2, 2
    c) 2, 8, 2
    d) 2, 2, 8

  3. Isotopes of an element have:
    a) Same physical properties
    b) Different chemical properties
    c) Different number of neutrons
    d) Different atomic numbers

  4. The number of valence electrons in a Chloride ion (Cl⁻) is: (Atomic number of Cl = 17)
    a) 16
    b) 17
    c) 8
    d) 7

  5. Which of the following atomic models is often referred to as the 'plum pudding' model?
    a) Rutherford's model
    b) Bohr's model
    c) Thomson's model
    d) Dalton's atomic theory

  6. An atom with atomic number 15 and mass number 31 contains:
    a) 15 protons, 16 neutrons, 15 electrons
    b) 16 protons, 15 neutrons, 15 electrons
    c) 15 protons, 16 neutrons, 16 electrons
    d) 31 protons, 15 neutrons, 15 electrons

  7. Which postulate of Bohr's model addressed the instability issue of Rutherford's model?
    a) Presence of a nucleus
    b) Electrons revolve in discrete orbits without radiating energy
    c) The atom is electrically neutral
    d) Most of the space in an atom is empty

  8. Identify the pair of isobars from the following:
    a) ¹²₆C and ¹⁴₆C
    b) ³⁵₁₇Cl and ³⁷₁₇Cl
    c) ¹₁H and ²₁H
    d) ⁴⁰₁₈Ar and ⁴⁰₂₀Ca

  9. What is the maximum number of electrons that can be accommodated in the L shell (n=2)?
    a) 2
    b) 8
    c) 18
    d) 32

  10. The subatomic particle discovered by James Chadwick is:
    a) Electron
    b) Proton
    c) Nucleus
    d) Neutron

Answers to MCQs:

  1. c) Atomic Nucleus
  2. c) 2, 8, 2
  3. c) Different number of neutrons
  4. c) 8 (Neutral Cl: 2,8,7. Cl⁻ ion gains one electron: 2,8,8. Valence electrons = 8)
  5. c) Thomson's model
  6. a) 15 protons, 16 neutrons, 15 electrons (Neutrons = A - Z = 31 - 15 = 16. Electrons = Protons for neutral atom)
  7. b) Electrons revolve in discrete orbits without radiating energy
  8. d) ⁴⁰₁₈Ar and ⁴⁰₂₀Ca (Same mass number A=40, different atomic numbers Z=18 and Z=20)
  9. b) 8 (Using 2n², 2(2)² = 8)
  10. d) Neutron

Revise these notes thoroughly. Understanding the progression of atomic models and the definitions of key terms like atomic number, mass number, isotopes, and isobars is essential. Good luck with your preparation!

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