Notes

when a high electric voltage of 10,000 volts or more is applied to the electrode of a discharge tube containing air or any gas at atmospheric pressure ,no electricity flows through the air .however ,when the high voltage of 10,000 volts is applied to the electrodes of discharge tube containing air or any gas at a very low pressure of about 0.001 mm of mercury , a greenish glow is observed on the wall of the discharge tube behind anode .This observation clearly show some invisible ray are called rays later ,they were named as electrons. At a time of j.j thomson ,only two fundamental particles (proton and electron) were known . In the year 1932,james chadwick discorverd another fundamental particle ,called neutron .but,the proper position of these particles in an atom was not clear till rutherford described the structure of atom . you will study about model in your higher classes .since hydrogen atom loses one elctron in its outermost orbit , its valency is taken as one and it is selected as the standard. valencies of the other elements are expressed in terms of hydrogen . thus , valency of an element can also be defined as the number.