The discovery of the atomic components

The basic components of the Atom:

1- Electrons:
The first evidence for the existence of subatomic particles came from studies of the conduction of electricity through gases at low pressures using cathode-ray-tube. The English physicist Joseph John Thomson was credited with the discovery and identification of the electron, and with the discovery of the first subatomic particle.

2- Protons and Neutrons; the Atomic Nucleus:
After J. J. Thomson discovered the negatively charged particles (electrons) in the atom, he proposed the structure of the atom.
His model stated that the atom consists of a positively charged sphere with the negatively charged electrons embedded in that sphere “plum pudding model.
A series of experiments carried out under the direction of Ernest Rutherford in 1911 lead to the discovery of the nucleus and its charge.
He and his students bombarded a piece of thin gold foil with α-particles (helium atoms minus their electrons). They used a fluorescent screen to observe the extent to which the α particles were scattered. Most of the particles went through the foil unchanged in direction; a few, however, were reflected back at acute angles.
He postulated that most the atom is an empty space, which explains why most of the bombarding particles passed through the gold foil un-deflected.  The reflection of α-particles was explained as the bombarding of the α particles to a positive nucleus.
After that, James Chadwick bombarded beryllium atoms with alpha particles which lead to the discovery of the nucleus.

By that scientists were able to propose a more correct model than the one proposed by J.J. Thomson.
In this model: 
1. The proton has a mass nearly equal to that of an ordinary hydrogen atom. The proton carries a unit positive charge (+1), equal in magnitude to that of the electron (-1).
2. The neutron is uncharged particle with a mass slightly greater than that of a proton.
Most of the mass (99.9%) is concentrated in the nucleus. Protons and neutrons are much heavier than electrons, even though the volume of the nucleus is much smaller than that of the atom.