Class 11 Chemistry: Structure of Atom

Review of Previous Class

• Covered discovery of electrons.
• Discussed charge to mass ratio of electron.
• Key values:
  • Charge of an electron (found by Millikan's oil drop method)
  • Mass of an electron
  • Charge by mass ratio

Current Class Topics

• Discovery of Protons
• Discovery of Neutrons
• Application questions on charge by mass ratio

Discovery of Protons

• Discovered by: Goldstein
• Method: Modified cathode ray tube
• Process:
  • Glass tube with two electrodes (cathode connected to negative terminal, anode connected to positive terminal).
  • High voltage applied, low pressure maintained.
  • Hydrogen gas used; canal rays observed moving from anode to cathode.
  • Canal rays are positive because they deviate towards the negative side of electric and magnetic fields.
  • Protons: H+ ions moving from anode to cathode.
  • Properties of Canal Rays: Depend on nature of the gas used (different gases produce different positive ions).

Charge to Mass Ratio

• Electrons: Constant value (1.75 x 10^11 C/kg)
• Protons (H+ ions): Depends on the gas, not constant like electrons
• Important Point: Higher charge to mass ratio is observed in particles with lower mass (e.g. electrons have the highest ratio because of their low mass).

Discovery of Neutrons

• Discovered by: Chadwick
• Method: Bombarding alpha particles on beryllium.
• Characteristics: Neutral charge, recently discovered (20th century).

Charge and Mass of Subatomic Particles

• Charge in Coulombs:
  • Electron: -1.6022 x 10^-19 C
  • Proton: +1.6022 x 10^-19 C
  • Neutron: 0 C
• Relative Charge:
  • Electron: -1
  • Proton: +1
  • Neutron: 0
• Mass in kg:
  • Electron: 9.1 x 10^-31 kg
  • Proton: 1.672 x 10^-27 kg
  • Neutron: 1.674 x 10^-27 kg
• Mass in AMU:
  • Electron: 0.0005
  • Proton: 1.007
  • Neutron: 1.008

Important Concepts

• Properties of canal rays depend on the gas.
• Charge to mass ratio concept.
• Particle with least mass has the highest charge to mass ratio.

Sample Question

• Question: Which has the highest charge to mass ratio?
  • Options: A) Electrons B) Protons C) Neutrons D) Alpha particles
  • Answer: Electrons
  • Reasoning: Electron's charge to mass ratio is constant and high.

Next Class

• Discussion on various atomic models.
• Topics: Thompson's atomic model, Rutherford's atomic model.
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