Two different isotopes of the same atom have

a. the same number of protons
b. the same number of neutrons
c. a different atomic number
d. the same mass

same atomic no. or no. of protons

An isotope of an element is defined as an atom with the same number of protons and electrons so it must have a different number of neutrons. Any changes in the number of neutrons will change the atomic mass of the atom. All forms of an element have the same atomic number. The answer is a. and c.