Two isotopes of chlorine have different mass numbers because they have different numbers of neutrons in their nuclei.

Here's a breakdown:

* Isotopes are atoms of the same element that have the same number of protons (which defines the element) but a different number of neutrons.

* Mass number represents the total number of protons and neutrons in an atom's nucleus.

Chlorine has two main isotopes:

* Chlorine-35 (³⁵Cl): Has 17 protons and 18 neutrons (17 + 18 = 35)

* Chlorine-37 (³⁷Cl): Has 17 protons and 20 neutrons (17 + 20 = 37)

Both isotopes have the same number of protons (17), making them chlorine atoms. However, the difference in their neutron count leads to a difference in their mass numbers.