Question

give similarity and one difference between a pair of isotopes

Answer 1

Isotopes of an element have same Atomic number i.e number of protons.
But they have different mass number i.e differ in number of neutrons.

Answer 2

Isotopes are the atom of the same element. They have same atomic number but different mass number. For example hydrogen has three isotopes - protium, deuterium and tritium, each having same atomic no. 1 but different mass no. 1, 2 and 3.