A natural element consists of two isotopes: Cl-35 and Cl-37. The composition of these two isotopes differs by:

There are no options in this question, however, it can be answered based on general understanding

- The number of neutrons each isotope contain

Explanation:

Isotopes are atoms of an element with the same atomic number or number of protons but different mass number/atomic masses. Since isotopes have same proton numbers, they have similar chemical behavior or identity.

However, difference in atomic mass or mass number of the same atomic number indicates that the number of neutrons each isotope contain differs from one another. Hence, in two isotopes of chlorine given as follows: Cl-35 and Cl-37, the NUMBER OF NEUTRONS in each atom differentiates the two isotopes.

Cl-35 contains 18 neutrons while Cl-37 contains 20 neutrons.