Which pairs of figures represent isotopes of the same element?

Isotopes of the same element have the same number of protons, which defines the element, but different numbers of neutrons, leading to different atomic masses. To determine which pairs of figures represent isotopes, one would look for figures that have the same number of protons (indicating they are the same element) and differing neutron counts.

In this scenario, figures I and III must have the same proton count, confirming that they represent the same element. If they differ in neutron count, they qualify as isotopes. This characteristic of isotopes is crucial in understanding nuclear properties, stability, and the concept of atomic mass variations among elements. Therefore, the identification of figures I and III as isotopes reinforces the concept of isotopes being forms of the same element due to their identical proton number and varying neutron numbers, justifying the selection of this answer.