CBSE · Class 12 · Physics · AtomsDiscuss the Alpha-particle scattering experiment (Geiger-Marsden experiment) in detail. Describe its experimental setup, observations, and the profound conclusions drawn from it regarding the structure of the atom.

Introduction to the Alpha-Particle Scattering Experiment

\nThe Alpha-particle scattering experiment, famously performed by Hans Geiger and Ernest Marsden under the guidance of Ernest Rutherford in 1911, stands as one of the most monumental milestones in modern physics. Prior to this experiment, J.J. Thomson's "Plum Pudding Model" was widely accepted, which proposed that an atom is a uniform sphere of positive charge with electrons embedded inside it like plums in a pudding. Rutherford's experiment completely dismantled this model and laid the foundation for the nuclear model of the atom.

Experimental Setup

\nThe experimental arrangement consisted of the following key components enclosed in an evacuated chamber to prevent scattering of alpha particles by air molecules:

• Alpha Source: A radioactive substance (such as Bismuth-214 or Polonium) enclosed in a lead block container. This source continuously emitted high-energy alpha particles (${}_2^4\text{He}$ nuclei) moving at high speeds.
• Collimator: A fine beam of alpha particles was obtained by passing the emitted particles through thick lead plates containing a narrow slit, producing a narrow, parallel beam of alpha particles.
• Gold Foil: A very thin sheet of gold foil (approximately $100 \text{ nm}$ thick, containing roughly $400$ layers of atoms) was placed in the path of the collimated alpha beam. Gold was chosen because it can be beaten into extremely thin foils, minimizing multiple scatterings within the target.
• Detector System: A moveable zinc sulfide ($\text{ZnS}$) fluorescent screen coupled with a microscope. Whenever a high-energy alpha particle struck the $\text{ZnS}$ screen, a tiny flash of light (scintillation) was produced. The microscope was used to count these scintillations at various angles relative to the incident beam.

Observations

\nWhen the experiment was conducted, Geiger and Marsden recorded the number of alpha particles scattered at different angles ($\theta$). The key observations were:

• Most Particles Deflected Minimally: The vast majority of the alpha particles ($>99%$) passed straight through the gold foil with little to no deflection (deflection angles less than $1^\circ$).
• Moderate Deflections: A small fraction of the alpha particles were deflected through moderate angles.
• Large Deflections: Only a very small number of alpha particles (about $1$ in $8000$) suffered deflections greater than $90^\circ$.
• Extreme Backscattering: An extremely rare fraction of alpha particles (roughly $1$ in $20,000$ to $100,000$) bounced backward, retracing their path with a deflection angle of nearly $180^\circ$.

Conclusions Regarding Atomic Structure

\nBased on these remarkable observations, Rutherford drew several radical conclusions that reshaped our understanding of atomic architecture:

• Presence of Empty Space: Since the vast majority of alpha particles passed through the foil without any significant deviation, it can be concluded that most of the space inside an atom is completely empty.
• Concentrated Positive Charge (The Nucleus): The rare instances of large-angle deflections and complete rebound indicate that the positive charge and nearly the entire mass of the atom are not distributed uniformly (as Thomson suggested), but are intensely concentrated in an extremely small, dense region located at the center of the atom, termed the nucleus.
• Size of the Nucleus: Using kinetic energy calculations of the alpha particles and closest approach distances, Rutherford estimated the radius of the nucleus to be around $10^{-14} \text{ m}$ to $10^{-15} \text{ m}$, which is approximately $10,000$ to $100,000$ times smaller than the overall radius of the atom ($10^{-10} \text{ m}$). This proves that the atom is mostly empty space with a microscopic, heavy core.
• Nuclear Force / Coulombic Repulsion: The deflection of positively charged alpha particles is caused by the strong electrostatic (Coulombic) repulsion exerted by the positively charged nucleus.