Rutherford scattering
Nearly all pass straight through; a very few bounce back.
Rutherford bombarded gold foil with α particles: the vast majority passed through almost undeflected, a few were deflected through large angles, and a very few even bounced back. This showed that the atom's positive charge and nearly all of its mass are concentrated in a tiny nucleus. The smaller the impact parameter b, the stronger the Coulomb repulsion and the larger the deflection angle.
Deflection angle vs. impact parameter
At the vertical line b = d/2 and the deflection is exactly 90°; for slightly larger b the deflection is small.
Think about it
At what impact parameter is the α particle deflected by exactly 90°?
b = d/2. From tan(θ/2) = d/(2b), when θ = 90°, tan 45° = 1. For 5 MeV α particles striking gold nuclei, d ≈ 45 fm, so b is about 23 fm, far smaller than the size of an atom. That is why large-angle scattering is so rare.
Model assumptionsNucleus at rest · Coulomb repulsion only · Trajectories from numerical integration · View spans ±600 fm
Data log
Set the parameters and click “Record data”. Record several rows with different parameters to plot their relationship and fit a straight line; pause and record at different times to study how a reading changes with time.