• $summary_{type}$: 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$\frac{theta}{2}$)/(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$$\frac{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.