I. Atomic Models Evolution

A. Dalton's Model (1803)

• Main postulates:
  • a. Elements composed of indivisible atoms
  • b. Atoms of same element are identical
  • c. Atoms combine, separate, or rearrange in chemical reactions
  • d. Atoms cannot be created or destroyed

B. Rutherford's Model (1911)

• Gold foil experiment findings:
  • a. Most space in atom is empty
  • b. Positive charge concentrated in nucleus
  • c. Electrons revolve around nucleus in orbits
• Defects:
  • a. Contradicts classical physics (continuous energy emission)
  • b. Predicts continuous spectrum instead of observed line spectrum

C. Bohr's Model (1913)

• Key postulates:
  • a. Electrons in fixed energy orbits
  • b. Energy proportional to distance from nucleus
  • c. Quantized angular momentum
  • d. Light absorbed/emitted during electron transitions
• Limitation: Does not depict 3D aspect of atom

D. Quantum Mechanical Model

• Current model based on quantum mechanics
• Defines electron probability distributions (orbitals)
• Explains complex atomic phenomena

II. Important Atomic Concepts

A. Proton Number/Atomic Number

• Number of protons in nucleus
• Unique to each element
• Used for arranging elements in periodic table

B. Nucleon Number/Atomic Mass

• Sum of protons and neutrons in nucleus

C. Isotopes

• Same element, different neutron numbers
• Affect molecular mass but not chemical properties
• Applications: Carbon dating, medical imaging

D. Ion Formation

• Cations (positive) and anions (negative)

E. Relative Atomic Mass

• Average mass of isotopes compared to 1/12 of carbon-12