How are isotopes of the same chemical element alike? how are they different?

The atomic mass of these species is different and atomic number remains same.

Explanation:

Isotopes are the chemical species of the same element having different number of neutrons.

Atomic Number = Number of electrons = Number of protons

Atomic Mass = Number of protons + Number of neutrons

For isotopes, as the number of neutrons differ, the atomic mass also differs.

For Example: Carbon has 3 naturally occurring isotopes: . The atomic number remains the same but atomic mass differs.

Hence, for isotopes, the atomic mass of these species is different and atomic number remains same.