Electrons are essential components of an element and decide its reactivity and bonding. They have their own unique properties known as quantum numbers which makes them all unique, existing in a number of sub orbitals around a nucleus.
The work of J.J Thomson’s student, Ernest Rutherford, led to the discovery of the Proton. Working with alpha particles fired at a piece of gold foil it was observed that instead of passing straight through it was scattered. Suggesting there was something large in the centre of the atom.