Long time this question remained open for scientists in spite of the fact that existence of atoms was predicted still by the Ancient Greek scientist Democritus. Last century the standard model of atom is developed.
Experiments of Rezeford
Experiments of the great scientist, "father" of modern nuclear physics, helped to create planetary model of atom. According to it, atom represents a kernel around which on orbits electrons rotate. The Danish physicist Niels Bohr finished this model within quantum representations a little. It turns out that an electron - one of the particles which are a part of atom.
This particle was opened by J.J. Thomson (lord Calvin) in 1897 in experiments with cathode beams. The great scientist found out that when passing electric current through the container with gas in it the negatively charged particles subsequently called electrons are formed.
Electron - the smallest particle having a negative charge. It does it stable (time of life of an order of the iott of years). Its state is described by several quantum numbers the Electron has own mechanical moment - to backs which can accept values +1/2 and-1/2 (spin quantum number). Existence a back was confirmed in Ulenbek and Gaudsmit's experiments. This particle submit to the principle of Paulie according to which two electrons cannot have the same quantum numbers at the same time, that is cannot be in identical quantum states at the same time. By this principle electronic orbitals of atoms are filled.
Proton and neutron
The kernel, according to the accepted planetary model, consists of protons and neutrons. These particles have almost identical weight, but at a proton the positive charge, a neutron has no it at all. The proton was opened by Ernest Rutherford as a result of his experiments with alpha particles which it bombarded gold atoms. The mass of a proton was counted. It appeared almost in 2000 times more the mass of an electron. A proton - the most stable particle in the Universe. Scientists consider that time of her life gets closer to infinity. The hypothesis of existence of a neutron was stated still by Rutherford, but experimentally he could not confirm it. It was made by J. Chadwick in 1932. The neutron "lives" about 900 seconds. Through this time the neutron will break up to a proton, an electron and an electronic neutrino. He is capable to cause nuclear reactions as can easily get into a kernel, passing action of forces of electrostatic interaction and to cause its division.
Both the proton, and neutron are not complete particles. According to modern representations, they consist of groups of quarks which connect them in a kernel. Quarks carry out strong and nuclear interaction between kernel components.