At first, the concept of isotopes was introduced by Frederick Soddy in 1913. The term isotope is associated with radioactive decay. This radioactivity led to the radical modifications in Dalton’s atomic theory.
There were two independent lines of the research which became evidence for the existence of isotopes. The most important study for the discovery of isotopes was radioactivity. By the year 1910, it became clear that some processes are associated with the phenomenon of radioactivity and it could cause the transformation of one element to another element. For instance, some ores having radioactive elements such as Thorium and Uranium contained the small quantities of radioactive substances and earlier this phenomenon was not studied. They regarded these substances as elements and gave them some special and unique names. For example, ionium was yielded from uranium ores and mesothorium from thorium ores. Chemically mesothorium was not distinguished from the radium.
The chemists always use a specific criterion to distinguish the elements from each other so based on that they concluded mesothorium and ionium are not the new elements but these are the modifying form of the elements they are originating from after radioactive decay. So, by using different data on this aspect it was generalized in 1910 by an English Chemist Frederick Soddy that element having different atomic weights can possess the identical chemical properties so they do belong to the same place in the periodic table.
With considerable precision, the scope of the conclusion was extended for not only including radioactive species but the stable elements also. After a few years, a comparison of atomic masses of stable elements was published by the Soddy. He was expecting a difference as the decay of thorium and uranium turned them to the different isotopes of the lead element. The atomic mass of the lead obtained from the uranium-rich ore was average 206.08 as compare to the lead obtained from the thorium that is 207.69 so, Soddy’s conclusion was verified in this regard.
After a few years, the mass spectrograph was developed by Francis William Aston. He was a student in the laboratory of Thomson and he has learned that two positive rays are produced by the gaseous element neon. Ions present in the heavy ray have 10% greater masses than the ions present in the lighter ray. To study these heavy and lighter rays, he constructed his equipment that was most precise than any other equipment present at that time. By the year of 1919, he argued and convinced the existence of neon 22 and neon 20. In the coming years, this information was rapidly accumulated in the laboratories and 1935 principal isotopes and the relative proportions of principal isotopes were known for the handful of the elements.
This discovery of isotopes brought the concept that chemical elements can be broken into the isolated and the small components which then can be used for various purposes. Nowadays, isotopes are commonly used in scientific experiments and it is considered as an advent ushered revolution in chemistry.