General Chemistry I - Lecture 1: Models of Atom

General Chemistry I - Lecture 1: Models of Atom

• Lecture delivered by Dr. Mahmoud A. A. Ibrahim, Chemistry Department, Faculty of Science, Minia University
• Lecture number 1 covers atomic models
• Models of the atom include Thomson's, Rutherford's, and Bohr's models

Thomson's Model

• Developed in 1911, after the components of the atom (protons and electrons) were discovered
• Proposed a model where protons and electrons are mixed uniformly throughout the atom

Rutherford's Model

• Rutherford tested Thomson's hypothesis with the gold foil experiment
• Bombardment of a gold foil with alpha particles (Helium nuclei)
• Observations:
  • Most alpha particles passed straight through the foil
  • Some alpha particles were deflected slightly
  • Very few alpha particles were deflected greatly
  • Even fewer bounced off the foil and back
• Conclusions:
  • The atom is mostly empty space
  • The nucleus is small, dense, and positively charged
  • The nucleus contains most of the atom's mass, and is 100,000 times smaller than the atom

Rutherford's Model Difficulty

• The model failed to explain the stability of atoms and atomic spectra from the classical viewpoint

Bohr's Model

• Developed two years after Rutherford's model
• Proposed that electrons are in stationary states of motion with fixed energies
• Electrons do not radiate when in these stationary states
• When an electron changes energy levels, it emits or absorbs a quantum of radiation, equal to the difference in energy between the levels
• Electrons orbit the nucleus in circular orbits
• Angular momentum of electrons is an integral multiple of h/2π
• Bohr's postulates, while initially accepted, are partly or entirely incorrect by modern quantum theory standards

Bohr's Model Derivation of Energy States

• The derivation of energy state expressions is straightforward
• Mechanical stability requires balancing Coulombic and centrifugal forces
• Coulombic force = Centrifugal force = (Ze²)/(4πε₀r²) = mv²/r