Chemistry: Historical Atomic Theory

Introduction

• Importance of understanding the history of atomic theory.
• Recommendation to view videos on historical atomic theory available in the online learning management system for better understanding.

Key Historical Figures and Concepts

Democritus (460-370 BC)

• Proposed that everything is composed of atoms.
• Believed atoms were indestructible and indivisible.

Robert Boyle

• Known for Boyle’s Law (gas law).
• Helped define the concept of an element.
• Emphasized experimentation and empirical observation (e.g., "The Sceptical Chymist" 1661).
• Pushed for theories to be based on experiments rather than matching experiments to preexisting theories.

Antoine Lavoisier

• Known for the Law of Conservation of Mass.
• Demonstrated that mass is conserved in chemical reactions.
• Conducted significant work on combustion reactions.

Joseph Proust

• Known for the Law of Definite Proportions.
• Found that substances have a consistent composition, regardless of how they are made.

John Dalton

• Known for the Law of Multiple Proportions and Dalton’s Atomic Theory.
• Proposed that all atoms of a given element are identical.
• Highlighted that atoms of different elements vary in mass and size.
• Introduced the concept that reactants combine in whole number ratios.

Atomic Models

Dalton's Atomic Theory

• Atoms are indivisible and identical for a given element.
• Recognized differences between atoms of different elements.
• Drawback: atoms are divisible into subatomic particles (protons, neutrons, electrons).

Thomson's Plum Pudding Model

• Discovered electrons.
• Model described atoms as a mass of positive charge with scattered electrons (similar to a chocolate chip cookie).
• Lacked a nucleus.

Rutherford's Gold Foil Experiment

• Demonstrated that most of an atom is empty space.
• Discovered the nucleus with positive charge through deflection of alpha particles.
• Did not explain why electrons orbit the nucleus.

Bohr's Model

• Proposed electrons move in fixed orbits with quantized energy levels.
• Explained emission spectra.
• Model had limitations with heavier atoms.

Schrödinger's Quantum Mechanical Model

• Electrons move in wave patterns, not fixed paths.
• Introduced the concept of orbitals, clouds of probability for electron locations.
• Most widely accepted model due to accuracy.

Laws of Proportions

Law of Definite Proportions

• Consistency in the composition of a compound regardless of sample size.
• Example: Sodium Chloride (NaCl) maintains a constant ratio of sodium to chloride.

Law of Multiple Proportions

• Different compounds can form with the same elements in different ratios.
• Example: Carbon with oxygen forms CO and CO2.

Conclusion

• Understanding of atomic theory includes laws of multiple and definite proportions and conservation of mass.
• Emphasized the importance of reviewing historical atomic theory videos for visual understanding.
• These concepts will be revisited throughout chemistry courses in Gen Chem 1 and 2.