Chemistry · Ch 4 — Structure of Atom
Discovery of Neutron
Discovery of Neutron
Rutherford's nuclear model left a puzzle: the measured mass of most atoms was roughly double what their proton count alone could account for, implying the nucleus must also contain some additional, electrically neutral, massive particle. Rutherford predicted the existence of such a particle in 1920, but it took until 1932 for James Chadwick to confirm it experimentally. Chadwick directed α-particles at a beryllium target, which emitted a highly penetrating but electrically neutral form of radiation; when this radiation was allowed to strike a block of paraffin wax (rich in hydrogen), it knocked protons out of the paraffin. By measuring the velocity of these ejected protons, Chadwick worked out the mass of the particle that had knocked them loose, and found it to be almost exactly the same as the mass …
What this figure shows. A diagram of Chadwick's 1932 apparatus: a radioactive α-source directed at a beryllium target, which emits an unknown, highly penetrating, electrically neutral radiation. This radiation is shown striking a block of paraffin wax, knocking protons out of the paraffin's hydrogen-rich material; a detector positioned to catch these ejected protons lets their velocity (and hence the mass of the neutral particle that knocked them out) be measured, which is how Chadwick showed the unknown radiation consisted of parti …