Discovery of Proton

In 1911, Ernest Rutherford investigated the structure of the atom by firing a narrow beam of positively-charged α-particles at an extremely thin sheet of gold foil, with a fluorescent screen arranged around the foil to record where each particle landed after passing through. Most α-particles passed straight through the foil as if nothing were there, a smaller fraction were deflected through small angles, and — the genuinely surprising result — a very small number bounced almost straight back the way they had come. Rutherford reasoned that such large deflections could only happen if the α-particles were occasionally colliding with something extremely small, extremely dense, and positively charged (since like charges repel); everything else about the result implied that atoms are mostly empty space. This experiment established that an atom has a tiny, massive, positively-charged nucleus at its centre. Rutherford followed this up in 1919 by showing that bombarding nitrogen gas with fast α-particles converts it into oxygen while simultaneously releasing hydrogen nuclei: $^{14}_{7}N + ^{4}_{2}\alpha \rightarrow\, ^{17}_{8}O + ^{1}_{1}H$. Because this hydrogen nucleus turned up as a product of transmuting several different elements, Rutherford proposed that every atomic nucleus contains hydrogen nuclei as one of its building blocks, …