short description of elementary particles

Elementary particles: The particles which are structureless, indivisible and not regarded as made up of some other particles are called elementary particles. Hundreds of elementary particles are discovered so far. These particles are fundamental in the sense that that these particles cannot be explained as system of other particles. A brief description of some particles is as follows:

1.       Electron : It is the first fundamental particle which was discovered by Thomson in 1897. It revolves around the nucleus of an atom in the different orbits. Electron plays an important role in explaining the physical and chemical properties of substances. Its charge is -1.6*10⁻¹⁹c and mass 9.1*10⁻³¹ kg. its symbol is e or ₋₁eᴼ.

2.       Proton : it was discovered by Rutherford in 1911. It has a positive charge (+1.6*10⁻¹⁹c) equal to the electronic charge and its mass (1.6726*10⁻²⁷kg) is 1836 times the electronic mass. In free state, the proton is a stable particle. Its symbol is p. it is also written as ₁H¹.

3.       Neutron: it is discovered by chadwick in 1932. It carries no charge. Its mass (1.6749*10⁻²⁷kg) is 1839 times the electronic mass. In free state the neutron is unstable (its mean life is about 17 minutes), but it constitutes a stable nucleus along with proton. Its symbol is n or ₀n¹.

4.       Positron: it was also discovered in 1932 by Anderson. Its charge and mass as same as those of electron, the only difference being that it is positively charged whereas the electron is negatively charged. Its symbol is e˖ or ₁eᴼ.

5.       Antiproton: it was discovered in 1955. Its charge and mass are same as those of proton, the only difference being that it is negatively charged. Its symbol is p̅.

6.       Antineutron: it was discovered in 1956. It has no charge and its mass is equal to the mass of neutron. The only difference between neutron and antineutron is that if they spin in the same direction, their magnetic moments will be in opposite directions. The symbol for antineutron is n̅.

7.       Neutrino and Anti-neutrino: The existence of these particles was predicted in 1930 by pauli while explaining the emission of β-particles of radioactive nuclei, but they were observed experimentally in 1956. Their rest mass and charge both zero but they have energy and momentum. Both neutrino and anti-neutrino are stable particles. The only difference between them is that their spins are in opposite directions. Their symbols are v and v̅ respectively.

8.       Pi-mesons: The existence of these particles are predicted by Yukawa in 1935 as originator of exchange-forces between the nucleons, but they were actually discovered in 1947 in cosmic rays. Pi-mesons are of three types:

                                I.            Positive pi-mesons

                              II.            Negative pi-mesons

                            III.            Neutral pi-measons