Atomic structure

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Who was the scientist responsible for proving the existence of the neutron? And what is the reaction in nuclide form, that was used to prove it?

James Chadwick discovered the neutron and presented his findings in 1932.
 
Until 1930 the only “primary” particles which had been observed were protons and electrons. Neutral particles had been detected, but it was assumed that these must be a combination of a proton and electron tightly bound together in some way, and it was not suspected that these were primary particles in their own right.
 
In 1930 two scientists (Bothe and Becker) discovered that in the bombardment of beryllium by alpha-particles, a penetrating radiation was emitted which was uncharged, but which nevertheless appeared to consist of particles. It was Chadwick who used this reaction to show that these neutral particles were primary particles, which he termed neutrons:
 
₄⁹Be + ₂⁴He –> ₆¹²C + ₀¹n
 
₂⁴He is an a-particle.
 
₀¹n represents a neutron.
 
Chadwick subsequently carried out reactions with beryllium and deuterium ( ₁²H) and with a-particles and boron-10 (₅¹⁰B), but I’ll let you research these for yourself.
 
On the internet, if you enter “Chadwick+neutron” (omit the inverted commas) in the search field on the website http://www.google.co.uk you will be presented with many articles linked with your question. In particular, at the head of the list, are Chadwick’s two original articles (modestly) presenting his ideas to the scientific world just over 70 years ago. These are both extremely illuminating pieces of writing. I suggest you download them, print them off and read them through thoroughly. They’re excellent. An alternative, but brief introduction is an article at http://www.pbs.org/wgbh/aso/databank/entries/dp32ne.html
 

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updated: 21 August 2003