Upper secondary · 15 min

Rutherford scattering: probing the atom with α particles

Fire a collimated α beam at a gold foil and trace the trajectories: nearly all pass through, a few deflect, and a rare few bounce straight back.

Goal

Reconstruct how the back-scattering of a few α particles forced the conclusion that almost all atomic mass sits in a tiny positive nucleus.

Apparatus and reagents

An α source shielded in a lead block with a narrow exit channel, a gold foil only ~0.4 µm thick, a zinc-sulfide screen that flashes on each hit, all inside vacuum.

Procedure

Follow the thick red beam line from the source to the foil and onward to the screen — this is what happens to most α particles.
Find the two thinner lines leaving the foil at an angle — α particles that passed near a nucleus and deflected.
Find the line that turns back toward the source — the 1-in-8000 event that shocked Rutherford.
Compare the relative thickness of the three kinds of line to judge how common each outcome is.

What to observe

The overwhelming majority of α particles go straight through — atoms are mostly empty space.
A small fraction deflects by a few degrees — close encounters with something small and charged.
Back-scattering exists at all only if the positive charge and mass are concentrated in a region far smaller than the atom.

Explanation

In the plum-pudding model the positive charge was spread over the whole atom, so a heavy, fast α particle could never bounce back from a thin foil. The observed ~1/8000 back-scattered fraction forced Rutherford’s 1911 model: a tiny nucleus ( $\sim10^{-14}$ m, versus $\sim10^{-10}$ m for the atom) carrying all the positive charge and nearly all the mass. The same nuclear picture later explained isotopes — nuclei with equal charge Z but different neutron numbers, like the $^{235}\ce{U}$ / $^{238}\ce{U}$ pair — and framed all of α, β and γ decay as processes inside that nucleus.

Chemists behind it

Ernest Rutherford