ATOM : atom is the smallest indivisible particle of the matter. Atom is made of electron, proton and neurons.
Electrons were discovered using cathode ray discharge tube experiment. Nucleus was discovered by Rutherford in 1911.
Cathode ray discharge tube experiment: A cathode ray discharge tube made of glass is taken with two electrodes. At very low pressure and high voltage, current starts flowing through a stream of particles moving in the tube from cathode to anode. These rays were called cathode rays. When a perforated anode was taken, the cathode rays struck the other end of the glass tube at the fluorescent coating and a bright spot on the coating was developed.
Results of Rutherford experiments:
- Cathode rays consist of negatively charged electrons.
- Cathode rays themselves are not visible but their behavior can be observed with help of fluorescent or phosphorescent materials
- In absence of electrical or magnetic field cathode rays travel in straight linesd. In presence of electrical or magnetic field, behaviour of cathode rays is similar to that shown by electrons.
- The characteristics of the cathode rays do not depend upon the material of the electrodes and the nature of the gas present in the cathode ray tube.
Charge to mass ratio of an electron was determined by Thomson. The charge to mass ratio of an electron as 1.758820 x 1011. C kg-1 Charge on an electron was determined by R A Millikan by using an oil drop experiment. The value of the charge on an electron is -1.6 x 10-19C.
The mass on an electron was determined by combining the results of Thomson’s experiment and Millikan’s oil drop experiment. The mass of an electron was determined to be 9.1094 x 10-31kg.
Discovery of protons and canal rays:
Modified cathode ray tube experiment was carried out which led to the discovery of protons.