Chadwick's Discovery of The Neutron

This topic is part of the HSC Physics course under the section Structure of The Atom

HSC Physics Syllabus

  • investigate, assess and model the experimental evidence supporting the nuclear model of the atom, including:

– the Geiger-Marsden experiment
– Rutherford’s atomic model
– Chadwick’s discovery of the neutron (ACSPH026)

Discovery of the Proton and the Neutron

This video discusses experiments that led to the discovery of the proton and neutron.



  • Rutherford’s atomic model did not show what was inside the positive nucleus. Besides protons, he realised that there must be other particles because the mass number of an atom was always found to exceed its atomic number (number of protons).

Experiments that led to the discovery of the Neutron


When alpha particles were fired at a thin block of beryllium, a nuclear transmutation resulted in the production of neutrons.


Alpha particle and beryllium


However, the neutrons produced this way were initially hypothesised to be high-energy gamma radiation because they were unaffected by electric and magnetic fields. All particles known at the time (e.g. electrons, protons) were charged. When further experiments were conducted to investigate the nature of this radiation, the hypothesis was rejected. 


neutron, paraffin wax


The 'radiation' was projected onto a proton-rich paraffin block, causing protons to be emitted. Analysis of these protons' momentum and kinetic energies provided an estimation of the energy of the gamma radiation. However, the energies of alpha particles that caused the emission of gamma radiation were far too small to allow for this possibility without violating the law of conservation of energy. 



When this 'radiation' was used to irradiate metal surfaces, no photoelectric effect was produced. If this gamma radiation had sufficient energy to eject protons from the paraffin block, it should have been able to eject electrons as the latter would require much smaller amounts of energy (work function).

Chadwick's Experiment and Discovery of the Neutron


Chadwick conducted the same experiment using beryllium and paraffin block but provided a different interpretation. He claimed that this unknown radiation was actually neutral particles – neutrons. 

By applying the law of conservation of momentum and conservation of energy, Chadwick determined the mass of a neutron. Chadwick reasoned that a neutral particle could eject a proton from the paraffin by imparting its momentum onto it (this explanation accounted for the kinetic energies of protons measured in the experiment).

Using the kinetic energy and momentum of emitted protons, Chadwick showed that the mass of a neutron is slightly greater than that of a proton.

Chadwick's discovery of the neutron added to the understanding of the structure of the atom as the atomic mass is now accounted for. 


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