discovery of oxygen

Q. 8. How Thomson discovered 'isotopes' from the analysis of positive rays ? How did it clear the anomaly of fractional atomic masses? (1996, 98)Or How are the isotopes discovered with the help of analysis of the properties of positive rays? (2001,02)Ans. Isotopes : The atoms having different masses but possessing same atomic numbers (or same chemical properties) are called isotopes.Discovery of Isotopes : When Thomson was studying the mass spectrograph of neon gas, he found two parabolas. He explained this fact by saying that the neon gas contains two kinds of atoms which can not be separated chemically, meaning that, they possess same chemical properties or same atomic number but thier ions have different values of e/M. That means they have different masses. For the first time he used the word 'isotopes' for these atoms having same atomic number but different masses. In this way, the study of positive rays lead to the discovery of isotopes.Thomson also measured the intensities of two parabolas and stated that, the two isotopes of neon having atomic masses 20 and 22 are present in the neon in the ratio of 9:1. Therefore, average atomic mass of neon is given by9x20 + 1x22 =2Qi 2 9 + 1Thus discovery of isotopes in neon also cleared the anomaly of fractional atomic mass of neon. Later on Aston confirmed the existence of isotopes in chlorine also.