Structure of the Atom

• Introduction to Sub-atomic Particles

  • Charged Particles in Matter
    • Friction experiments (comb and dry hair)
    • Discovery of opposite charges
  • Electron
    • Discovered by J.J. Thomson (Cathode ray experiment)
    • Negatively charged ($e^-$)
    • Negligible mass
  • Proton
    • Discovered by Eugen Goldstein (Channel rays / Anode rays)
    • Positively charged ($p^+$)
    • Heavy mass compared to electron
  • Neutron
    • Discovered by James Chadwick
    • Neutral particle ($n$)
    • Mass nearly equal to proton

• Atomic Models

  • Thomson's Model of Atom
    • Plum Pudding or Watermelon model
    • Sphere of positive charge with embedded electrons
    • Neutral overall (equal positive and negative charge)
    • Limitation: Failed to explain experimental results of later scientists
  • Rutherford's Model of Atom
    • Alpha-particle scattering experiment (Gold foil)
    • Observations:
      • Most particles passed straight
      • Few deflected at small angles
      • Very few bounced back ($180^\circ$)
    • Conclusions:
      • Atom has a tiny, dense, positively charged nucleus
      • Most space inside the atom is empty
      • Electrons revolve around the nucleus in circular paths
    • Limitations:
      • Could not explain stability of the atom (charged particles losing energy while accelerating)
  • Bohr's Model of Atom
    • Proposed by Niels Bohr (1913)
    • Only certain special orbits known as discrete orbits of electrons are allowed
    • Electrons do not radiate energy while revolving in these orbits
    • Orbits called energy levels or shells ($K, L, M, N...$ or $n=1, 2, 3, 4...$)

• Distribution of Electrons in Orbits

  • Bohr-Bury Scheme
    • Maximum number of electrons in a shell formula: $2n^2$ ($n$ = orbit number)
    • Maximum capacity of outermost shell: 8 electrons
    • Shells are filled in a step-wise manner (lower energy first)
  • Valency
    • Combining capacity of an atom
    • Determined by the number of valence electrons (electrons in the outermost shell)
    • Octet rule: Atoms react to achieve 8 electrons in the valence shell (stability)

• Atomic Number and Mass Number

  • Atomic Number ($Z$)
    • Equal to the number of protons in the nucleus
    • Also equal to the number of electrons (in a neutral atom)
    • Defines the element
  • Mass Number ($A$)
    • Total number of protons and neutrons (nucleons) in the nucleus
    • $Mass\ Number = Protons + Neutrons$

• Isotopes and Isobars

  • Isotopes
    • Atoms of the same element with same atomic number ($Z$) but different mass numbers ($A$)
    • Example: Hydrogen ($Protium$, $Deuterium$, $Tritium$), Carbon ($^{12}C$, $^{14}C$)
    • Chemical properties are similar, physical properties differ
    • Applications:
      • Uranium isotope in nuclear reactors
      • Cobalt isotope in cancer treatment
      • Iodine isotope in goitre treatment
  • Isobars
    • Atoms of different elements with the same mass number ($A$) but different atomic numbers ($Z$)
    • Example: Calcium ($^{40}{20}Ca$) and Argon ($^{40}{18}Ar$)