The theory of Dalton did not hold long and it was proved through the experiments of research worker like JJ Thomson (1897), Rutherford (1911), Neils Bohr (1912), Vector Chidwick, Moseley etc., that atom is made up of smaller particles, called the subatomic particles like electron, proton, neutron, positron, neutrino meson etc. However, former three are considered as the fundamental particles and later are appeared only for a short instance during the microparticles exchange mechanism. Thus, these are not fundamental particles.
The Subatomic particles named for the weak force is the wboson
Electrons, protons, and neutrons are the fundamental particles of atom, discovery and properties of which are as follows.
a) Discovery of Electrons (-1eº) Electron was discovered in cathode rays experiments by JJ Thomson in 1877. In this experiment, when the pressure of a discharge tube maintained at high potential is reduced to 10-6 atm, a stream of negatively charged particles, called electrons, is originated from cathode. These rays were called cathode rays.
Characteristics of these rays and their particles are
- These rays themselves are not visible but their behaviour can be observed with the help of fluorescent or phosphorescent materials. (Note that television picture tubes are cathode ray tubes).
- In the presence of electrical or magnetic field, the behaviour of cathode rays are similar to that expected from negatively charged particles, called electrons.
- The carge on an elecctron, i.e., –1.602 x 10-19C was determined by Mulikan through oil drop experiment.
- Actual mass of an electron i.e., 9.11x 10-31kg was calculated by JJ Thomson. Of the three fundamental particles of an atom, electron is the lightest.
- e/m ratio (specific charge) of electrons was determined by Thomson as 1.76x 108 C/g.
- e/m ratio of the electron was found to be independent of the nature of gas and electrode used. Therefore, electrons are fundamental particles of all kinds of matter.
b) Discovery of Protons Even before the electron was identified, E Goldstein in 1886 discovered the presence of new radiation in a gas discharge and called them canal rays. These rays were positively charged radiations which ultimately led to the discovery of another sub-atomic particle. This sub-atomic particle had a charge, equal in magnitude but opposite in sign to that of the electron. It was given the name proton by Rutherford in 1919. Mass of proton is 1.67 x 10-27 kg while its charge is +1.6x 10-19 C.
c) Discovery of Neutrons The positive charge of a nucleus is due to the positvely charged particles called protons alone. There is another sub atomic particle called neutron, identified by Chadwick (1932) by bombarding a thin sheet of beryllium by α-particles. Neutrons are electrically neutral particles (i.e., have no charge) having a mass slightly greater than that of the protons. Mass of neutron is 1.67×10-27 kg (i.e., nearly equal to that of proton).
Particles other than electrons, protons and neutrons are called non-fundamental particles.
a) Positron: It was discovered by Anderson in 1932. It is the antiparticle of electron (i.d., its charge is positive and its mass is equal to that of the mass of electron). Its symbol is e-+.
b) Antiproton. It is the antiparticle of proton. It was discovered in 1955. Its charge is –e and its mass is equal to that of the mass of proton. Its symbol is p.
For every fundamental particle, there exists an identical fundamental particle just opposite in some property. It is called antiparticle of that fundamental particle. e.g., Electron and positron are identical in all respects, except that charge on them are opposite. So, positron is an antiparticle of electron.
c) Neutrino and Antineutrino The existence of these particles was predicted in 1930 by Pauli while explaining the emission of β-particles from radioactive nuclei, but these particles were actually observed experimentally in 1956. Their rest mass and charge both are zero, but they have energy and momentum. These are mutually antiparticles of each other.
d) Pi-mesons The existence of π-mesons was predicted by Yukawa in 1935, but they were actually discovered in 1947 in cosmic rays. Nuclear forces are explained by exchange of π mesons between the nucleons, Π-mesons are of three types- positive pi-meson, negative pi-mesons, and neutral pi-mesons. The mass of pi-mesons positive is 274 times the mass of the electron and Pi-neutral mesons nearly 264 times the electronic mass.
e) Quarks and Bosons: The elementary particles from which other heavy sub-atomic particles like protons, neutron, etc are formed, are called quarks. These particles carry fractional charge. Bosons are the particles for which number of rotations are whole number.