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Science: Class 9 — Chapter 8: Journey Inside the Atom

8.1 Rediscovering the Roots of Atomic Theory

8.1.1 Ancient Philosophical Perspectives

  • Acharya Kanada: Proposed that dividing matter (dravya) repeatedly eventually leads to indivisible particles called parmanus, which form dyads and triads to create the universe.
  • Leucippus and Democritus: Greek philosophers who proposed similar indivisible particles called atomos (meaning indivisible).

8.1.2 Dalton's Atomic Theory

  • John Dalton (1808): Formulated the first scientific atomic theory stating that all matter is composed of indivisible atoms, serving as fundamental building blocks.
8.2 A Short Historical Journey Through Atomic Models

8.2.1 Cathode Rays and Discovery of Electron

  • Radioactivity: The discovery that certain elements emit radiation proved that atoms are divisible into smaller components.
  • J. J. Thomson (1897): Discovered streams of negatively charged particles called electrons using cathode ray tube experiments.
  • Electron Charge: Defined as -1.602 × 10-19 C, taken as -1 by convention.

8.2.2 Thomson's Plum Pudding Model

  • Plum Pudding Model: Thomson described the atom as a sphere of positive charge with electrons distributed throughout it, similar to seeds in a watermelon.

8.2.3 The Gold Foil Experiment

  • Alpha Particles: Positively charged helium nuclei used as a beam by Geiger and Marsden under Ernest Rutherford.
  • α-ray Scattering: Most alpha particles passed straight through gold foil, but a small fraction deflected sharply or bounced back, contradicting Thomson's model.

8.2.4 Rutherford's Planetary Model

  • Nucleus: Rutherford concluded that the positive charge and mass of an atom are concentrated in an extremely dense, tiny central region.
  • Planetary Model: Model proposing that electrons revolve around the nucleus like planets orbiting the Sun.

8.2.5 Limitations of Rutherford's Model

  • Instability Dilemma: According to electromagnetic theory, accelerating electrons in a circular path should constantly lose energy and eventually spiral into the nucleus, causing the atom to collapse.

8.2.6 Discovery of the Proton

  • Proton: Discovered and named by Rutherford as the subatomic particle carrying a positive charge equal and opposite to the electron.

8.2.7 Bohr's Model of the Atom

  • Niels Bohr (1913): Postulated that electrons revolve around the nucleus in fixed circular paths called orbits, shells, or stationary states.
  • Energy Levels: Designated as K, L, M, N (n = 1, 2, 3, 4), where energy increases as distance from the nucleus increases.
  • Energy Transitions: Electrons do not radiate energy in stationary orbits, but transition between energy levels by absorbing or emitting discrete packets of energy.
8.3 Mass Constituents of an Atom

8.3.1 Discovery of the Neutron

  • James Chadwick (1932): Discovered the neutron, a neutral subatomic particle residing in the nucleus with mass nearly equal to that of a proton.
  • Nuclear Force: Neutrons minimize proton-proton electrostatic repulsion inside the nucleus, binding nucleons tightly via strong nuclear forces.
8.4 Symbols of Elements

8.4.1 Chemical Notation Evolution

  • IUPAC Rules: Element symbols are derived from English or Latin names (e.g., Fe for Ferrum, Na for Natrium). The first letter is uppercase and the second lowercase.
8.5 & 8.6 Atomic Identity: Numbers & Mass

8.5.1 Atomic Number (Z)

  • Atomic Number: Denoted by Z, representing the total number of protons in the nucleus, which uniquely identifies an element.

8.6.1 Mass Number (A)

  • Mass Number: Denoted by A, representing the total sum of protons and neutrons (nucleons) in the nucleus. Calculated as A = Z + Neutrons.
8.7 Electronic Configuration

8.7.1 Distribution of Electrons

  • Bohr-Bury Rules: The maximum number of electrons in any shell is given by 2n2 (K-shell: 2, L-shell: 8, M-shell: 18).
  • Stepwise Filling: Inner shells must be completely filled before outer shells begin filling, with the outermost shell capped at 8 electrons.
8.8 Combining Capacity of Atoms

8.8.1 Valence Electrons and Valency

  • Valence Shell: The outermost electron shell of an atom containing the valence electrons.
  • Octet Rule: Atoms interact to obtain a stable state of 8 electrons (or 2 for helium) in their outermost shell.
  • Valency: The combining capacity of an atom, determined by the number of electrons gained, lost, or shared to achieve a complete octet.
8.9 A Deeper Look into Atomic Structure

8.9.1 Isotopes

  • Isotopes: Atoms of the same element having identical atomic numbers (Z) but different mass numbers (A). Examples include Protium (1H), Deuterium (2H), and Tritium (3H).
  • Average Atomic Mass: The weighted average of the atomic masses of all naturally occurring isotopes based on their relative abundances (e.g., Chlorine is 35.5 u).

8.9.2 Practical Applications of Isotopes

  • Uranium-235: Fuel in nuclear fission reactors.
  • Cobalt-60: Cancer radiotherapy.
  • Iodine-131: Diagnosis and treatment of goitre.
  • Carbon-14: Radioactive dating of fossils.

8.9.3 Isobars

  • Isobars: Atoms of different elements that share the same mass number (A) but possess different atomic numbers (Z) (e.g., Argon-40, Potassium-40, Calcium-40).