Question

April 30th marks the 123rd anniversary since JJ Thomson announced to the Royal Institution of Great Britain that he had discovered something smaller than an atom. Some thought this was a prank announcement as the atom had been thought to be indivisible, which is why it was called the atom. Regardless, Thomson reported identifying a particle with a miniscule mass (relative to the atom) and a negative charge. What particle had Thomson discovered?

Answer 1

A small particle with a miniscule mass (relative to the atom) and negative charge. This type of particle is an electron.

As electron's mass is very small as compared to the atom and it has a negative charge on it.

Hence, the particle that Thomson discovered was an  electron.