Lecture Notes: Dalton's and Rutherford's Atomic Theories

Introduction

• Topic: Atomic and Molecular Structure
• Focus: Dalton's Atomic Theory & Rutherford's Atomic Model

Dalton's Atomic Theory

Key Postulates:

1. Smallest Particles
   • Elements are composed of tiny particles called atoms.
   • Atoms are indivisible and indestructible.
2. Identical Atoms
   • Atoms of the same element are identical in mass and properties.
   • Atoms of different elements have different masses and properties.
3. Atoms in Compounds
   • Atoms combine in fixed ratios to form compounds.
   • Fixed composition signifies specific compounds.
4. Conservation of Mass
   • Atoms cannot be created or destroyed in chemical reactions.
   • Based on Law of Conservation of Mass and Energy.

Rutherford's Atomic Model

Key Features:

1. Experimental Setup
   • Alpha particles directed at a thin gold foil.
   • Detection via a zinc sulfide screen around the foil.
2. Observations
   • Most alpha particles pass through the foil.
   • Some particles deflect or bounce back.
3. Conclusions
   • Existence of a small, dense, positively-charged nucleus.
   • Nucleus contains protons.
   • Electrons orbit the nucleus in specific paths (orbits).
   • Atom is mostly empty space.
   • Atom is electrically neutral (number of protons equals number of electrons).
   • Atomic mass concentrated in the nucleus (electrons have negligible mass).

Limitations of Rutherford's Model

1. Behavior of Electrons
   • Failed to explain electron behavior in orbits.
2. Stability of Atom
   • Failed to explain atom’s stability.
   • Predicted electrons spiraling into the nucleus due to energy loss through radiation.

Upcoming Topics

• Bohr's Atomic Model
• Overcoming Rutherford's limitations
• Detailed discussion on atomic numbers, mass numbers, and subatomic particles

Summary

• Dalton's theory laid foundational concepts.
• Rutherford discovered nucleus and proposed atomic structure, but had limitations.
• Bohr's model to address these issues next.