Prior to 1911, the structure of the atom was modeled by J.J. Thomson's "Plum Pudding Model," which envisioned a diffuse sphere of positive charge containing embedded negative electrons. The gold foil alpha scattering experiment conducted by Ernest Rutherford, Hans Geiger, and Ernest Marsden disproved this model and established the planetary nuclear model of the atom in Cambridge IGCSE Physics.

Experimental Setup

1. A narrow beam of energetic positively charged alpha particles (^{4}_{2}lpha) was directed at an extremely thin sheet of gold foil (a few hundred atoms thick) inside an evacuated vacuum chamber.
2. A movable zinc sulfide (ZnS) detector screen linked to a microscope was rotated around the foil to detect scattered alpha particles via light scintillations.

The experimental results overturned classical expectations and provided three key pieces of empirical evidence:

1. Observation 1: The Vast Majority of Alpha Particles Passed Straight Through Undeflected

• Result: Over $99\%$ of the incident alpha particles traveled through the gold foil in a straight line without changing direction.
• Theoretical Deduction: The atom consists mostly of empty space. The positive charge and mass are not spread uniformly throughout the atom's volume.

2. Observation 2: A Small Fraction of Alpha Particles Were Deflected Through Small Angles

• Result: A minor percentage of alpha particles experienced small angular deflections ($5^\circ ext{ to } 30^\circ$) away from their initial straight path.
• Theoretical Deduction: The nucleus is positively charged. Because alpha particles are positively charged ($+2e$), they experienced electrostatic (Coulomb) repulsive forces when passing close to the positive atomic core.

3. Observation 3: An Extremely Small Fraction Rebounded Backwards Through Large Angles ($>90^\circ$)

• Result: Approximately 1 in every 8,000 alpha particles was deflected backward by more than $90^\circ$, returning toward the alpha source.
• Theoretical Deduction: The nucleus is extremely small, dense, and contains almost all the mass of the atom. For an energetic alpha particle to be completely turned around, it must strike a concentrated mass much heavier than itself that occupies an extremely small fraction of the total atomic volume (roughly $1/10,000$th of the atomic diameter).

Rutherford's alpha scattering experiment proved that the atom consists of a tiny, extremely dense, positively charged nucleus containing almost all atomic mass, surrounded by mostly empty space occupied by orbiting electrons.