Answer:
1
Explanation:
Isotopy is the ability of elements to exist in various forms by having different mass numbers but the same atomic number. The isotope of hydrogen are protium, deuterium and tritium. The isotope mentioned in the question is tritium with mass number 3. mass number = atomic number +number of neutrons = 1+2=3.
Whichever the isotope of hydrogen, they all have the same atomic number which is 1.
