Atomic Structure : Discovery of Electrons, Protons & Neutrons

Intro: Atom is the smallest particle of a substance which is generally not found in the free state but takes part in a chemical reaction.

According to John Dalton’s hypothesis presented in 1808 the atom is not divisible. But, Michael Faraday, J J Thomson, Goldstein etc proved that atom is divisible and experiments performed by them shown that atom can be divided further into smaller particles. As explained below:

Discovery of Cathode rays & electrons: Michael Faraday performed an experiment by taking a gas at a very low pressure and at high voltage in a glass tube. He found that some rays are coming from Cathode and moving towards the anode. These rays were named as Cathode Rays which are made up of negatively charged particles called electrons.

Properties of cathode rays:

1.Cathode rays travel in a straight line.

2.Cathode rays contain negatively charged particles.

3.Cathode rays contain material particles.

4.Cathode rays affects photographic plates.

5.Cathode rays cause the ionization of the gas they pass through.

e/m ratio of an electron : J J Thomson was the first person to decide e/m ratio which was equal to 1.758820 × 10^11 C/kg.

Charge on an electron : The charge on an electeon was decided by the Oil Drop Experiment performed by R A Mullican. The charge on an electron was found -1.6 × 10^-19 C.

Mass of an electron : The mass of an electron can be calculated as follows:

m = e/(e/m) = 9.1 × 10^31 kg

◆Discovery of proton : After the discovery of electron, another particle was imagined which must be positively charged because an atom is neutral.

Goldstein got success in 1886 in discovering the positively charged particle which was named proton.

Experiment : A similar experiment as in case of electron was performed with perforated cathode this time. It was found that when the electric current of high voltage about 10000 volt was supplied some rays are coming towards cathode and striking the wall of the tube behind it. Since, these rays seem to come from anode so they were named anode rays. They are also called canal rays or positive rays.

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