Electrons, protons and which other sub-atomic particle were discovered in the 19th and early 20th century?

The electron, discovered in 1897 by J. J. Thomson, was the first subatomic particle to be observed. The nucleus of ordinary hydrogen was understood to be a single proton after the nucleus of the atom was discovered in 1911 by Ernest Rutherford. In 1932, a neutron was detected.

because u need two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei to form isotopes.

so since they all have the same shape? i mean maybe because m& m and reeses are both chocolates

with this one i know the answer in craters one reason i looked it up and i learned about it there is my evidence