Question

Isotopes are identified by their _______.

Answer 1

This is a phenomenon whereby atom of an element exhibit different mass number but have the same atomic number.

Answer 2

isotopes are the elements with same atomic but different mass number. For example

Isotopes of hydrogen......

Protium.....atomic no. 1 and atomic mass 1
Deutrium......atomic no. 1 and atomic mass 2
Tritinium.......atomic no. 1 and atomic mass 3

Isotopes are defined by their mass number.