Answer: It is so because they have different number of neutrons.
Explanation:
Isotopes are defined as the chemical species which have same number of protons but differ in the number of neutrons.
They have same atomic number but different atomic mass.
For Example: [tex]_6^{12}\textrm{C},_6^{13}\textrm{C}\text{ and }_6^{14}\textrm{C}[/tex] are the three isotopes of carbon.
The number of protons in all the three isotopes is same which is 6 and the number of neutrons in the three isotopes are 6, 7 and 8 respectively.
Hence, it is so because they have different number of neutrons.
