• summary_type: concept
• word_count: 170

Rutherford's alpha scattering experiment (Geiger-Marsden, 1911) established the nuclear model of the atom. Alpha particles directed at thin gold foil showed three key results: most passed straight through (atom is mostly empty space), some deflected at large angles (concentrated positive charge), and about 1 in 8000 bounced back beyond 90 degrees (tiny, dense nucleus). The impact parameter b = (kZe^2 cot(theta/2))/(1/2 mv^2) relates the perpendicular distance of approach to the scattering angle. The distance of closest approach r_0 = kZe^2/KE gives an upper bound for nuclear size (~10^-14 m for gold). The fraction of particles scattered beyond angle theta is proportional to Z^2 and cot^2(theta/2). Critical limitation: Rutherford's model couldn't explain atomic stability (accelerating electrons should radiate and spiral inward) or why atoms emit discrete spectral lines rather than continuous radiation. These failures motivated Bohr's quantum postulates.