Answer: They will have same number of protons.
Explanation:
Isotopes are defined as the chemical species which belong to the same element and have same atomic number but differ in their atomic masses.
This also means that they have same number of protons but have different number of neutrons.
For Example: Isotopes of carbon are [tex]_6^{12}\textrm{C},_6^{13}\textrm{C}\text{ and }_6^{14}\textrm{C}[/tex]
They all have 6 neutrons but have 6, 7 and 8 neutrons respectively.
Hence, two isotopes of the same element will always have same number of protons.
