Video Lecture 1: Thomson’s Plum Pudding & Rutherford’s Gold Foil Experiment

# Video Lecture 1: Thomson’s Plum Pudding & Rutherford’s Gold Foil Experiment
**Speaker:** Dr. Ananya Sharma | **Duration:** ~06:00 mins

### Introduction: Cracking Dalton’s Indivisible Atom
In 1808, John Dalton postulated that atoms were hard, indivisible spheres. But at the turn of the 20th century, physics shattered this view!
- In 1897, **J.J. Thomson** discovered the **Electron** ($e^-$) using cathode ray discharge tubes, showing it carried a negative charge and an infinitesimal mass ($1/2000$ of a hydrogen atom).
- In 1886, **E. Goldstein** discovered canal rays, identifying positively charged subatomic particles: **Protons** ($p^+$).

### J.J. Thomson’s Plum Pudding Model (1898)
Thomson proposed that an atom is a sphere of uniform positive charge embedded with negatively charged electrons—like plums in a pudding, or black seeds in a red watermelon!

### Rutherford’s Alpha-Particle Scattering Experiment (1911)
Ernest Rutherford tested Thomson's model by firing energetic, positively charged alpha particles ($He^{2+}$) at an ultra-thin gold foil (barely 1000 atoms thick):
- **Observation 1:** Most alpha particles passed straight through the gold foil without any deflection! $\rightarrow$ *Most of the space inside the atom is completely empty!*
- **Observation 2:** A small fraction were deflected by small and large angles. $\rightarrow$ *The positive charge is concentrated in a tiny fraction of volume.*
- **Observation 3:** Astonishingly, roughly 1 in 12,000 alpha particles rebounded straight backward by $180^\circ$!
Rutherford remarked: *"It was almost as incredible as if you fired a 15-inch artillery shell at a piece of tissue paper and it came back and hit you!"*

Rutherford deduced the existence of the **Atomic Nucleus**: an extremely dense, positively charged core at the center of the atom!