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Elementary particles of atom
The study of the structure of atom reveals that the fundamental particles electron, proton and neutron are the building blocks of an atom. But the extensive studies on cosmic rays have revealed the existence of numerous new nuclear particles like mesons. These particles are classified into four major groups as photons, leptons mesons and baryons.
Photon is a quantum of radiation with no charge and no mass, but it is a carrier of energy. It travels with velocity of light.
Leptons are lighter particles having mass equal to or less than about 207 times the mass of an electron except neutrino and antineutrino. This group contains particles such as electron, positron, neutrino, antineutrino, positive and negative muons. The electron and positron are the antiparticles. Neutrino and antineutrino are also associated with β-ray emission. The neutrinos and antineutrinos are massless and chargeless particles, but carrier of energy and spin. Muons were discovered in cosmic ray studies.
Mesons are fundamental particles carrying a single unit of charge and possessing mass intermediate between electron and proton (me and mp). The name meson was given by Yukawa in 1935. The three types of mesons are (i) π-meson (pion) (ii) K−meson (kaon) and (iii) η-meson. The mesons are the interaction agents between nucleons. The rest mass of mesons vary between 250 me and 1000 me.
Baryons form the heavier particle group. Protons and neutrons are called nucleons and the rest of the heavier particles other than nucleons are known as hyperons. There are four types of hyperons which are lambda, sigma, xi and omega hyperons. Protons and neutrons are around 1836 times the mass of the electron, whereas the mass of the hyperons vary from 2180 me and 3275 me.
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