ANSWER:
Isotopes & Isotopy:
“The atoms of the same element having same atomic number but different atomic masses are called isotopes, and the phenomenon is termed as isotopy, first discovered by Soddy.”
Isotopes possess same chemical properties and same position in the periodic table. They have same number of protons and electrons but differ only in the number of neutrons.
Examples: Carbon has three isotopes 6C12, 6C13 & 6C14, expressed as C–12, C–13 & C–14. Each of these have 6-protrons and 6 electrons. However, these isotopes have 6, 7 & 8 neutrons respectively. Similarly, hydrogen has three isotopes 1H1, 1H2 & 1H3 , called protium, deuterium and tritium.
(Ref. An Insight Into Objective Chemsitry-11, Ch-01, Q. No. 16)