Dalton’s Atomic Theory

In 1808 John Dalton published a new system of chemical philosophy in which he propose the following theory :-

  1. Matter consist of indivisible atoms.
  2. All the atoms of a given element have identical properties.
  3. Compounds are formed when atoms of different elements combined in a fixed ratio.
  4. Chemical reactions only involve the rearrangement of atoms that is the atoms are neither created nor destroyed During the during a chemical reaction.

Drawbacks of Dalton’s atomic theory:

  • It failed to describe the fact that atoms of same element had different atomic masses.
  • The theory was discarded after the discovery of sub atomic particles (electrons , protons and neutrons).

Subatomic Particles and Their Properties

The theory of Dalton was not applicable later as it was discovered that the subatomic particles  Exist in nature like the experiments of JJ Thomson in 1897, Rutherford  in 1911, , Niels Bohr in 1912, Henry Mosley etc. They proved that the atom is made up of small particles called the subatomic particles like electron, proton, neutron, positron, neutrino, mesons etc.

However the electrons, protons and neutrons are considered as the fundamental particles so let’s discuss about the fundamental particles

Fundamental particles

1 Discovery of electrons

Electron was discovered in cathode ray experiment by JJ Thomson in 1897. In this experiment when the pressure of a discharge tube maintained at high potential is reduced to 10-6atm, a stream of negatively charged particles called the electrons is originated from cathode. These rays were called cathode rays.

Characteristics of these rays and their particles Are

  1. The results themselves are not visible but their behavior can be observed with the help of fluorescent or phosphorescent materials.
  2. In the presence of electric or magnetic field the behavior of cathode rays are similar to that of expected from negatively charged particles called electrons.
  3. The charge on electron that is, -1.602×10-19C , was Determined by Mulliken through oil drop experiment.
  4. Actual mass of an electron that is 9.11×10-31kg was calculated by JJ Thomson. And it was seen that electron was lightest particle.
  5. e/m Ratio of electrons was determined by Thomson is 1.76×108C/gram.
  6. e/m ratio of electron was found to be independent of the nature of the gas and electrode used therefore electrons therefore electrons are fundamental particles of all kind of matter.

Discovery of protons

E.Goldstein in 1886 discovered the presence of new radiation in a gas discharge and called them Canal rays. These were positively charged radiations which ultimately led to the discovery of another subatomic particle. This subatomic particle had a charge equal in magnitude but opposite to that of electron it was given the name proton by Rutherford in 1919. Mass of proton is 1.67×10-27kg while it has charge +1.6×10-19 C.

Discovery of neutrons

The positive charge of nucleus is due to the positively charged particles called protons. But mass of the nucleus is not due to proton only there are some other subatomic particles also that he called the neutrons these were identified by J.Chadwick in 1932 by bombarding a thin sheet of beryllium by alpha particles . Neutrons are basically electrically neutral particles that have no charge having a mass slightly greater than that of the protons mass of neutron is a 1.67×10-27Kg nearly equal to that of a proton but not exactly equal to proton.

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