Which property of the isotopes must be different? a. the atomic number b. the electric charge c. the element name d. the mass number

D. The mass number

Explanation:

Isotopes are defined for same atoms

If an atom has same number of protons and same number of electrons but different number of neutrons in it then such pair of atoms is known as isotopes

so we have

same atomic number in all isotopes

since atomic number is same so we can say that total charge of the two atoms must be same

So we will say that since number of neutrons is different in isotopes to sum of neutrons and protons which is known as mass number is different in such pair of atoms

So correct answer is

D. The mass number