Final answer:
The concept of isotopes explains why Rhenium has an average atomic mass of 186, considering the relative abundances and masses of its naturally occurring isotopes.
Step-by-step explanation:
The element Rhenium has two naturally occurring isotopes, and its average atomic mass is reported to be 186. The concept of isotopes refers to different forms of an element that have the same number of protons but a different number of neutrons. Isotopic abundances contribute to the average atomic mass of an element, which is calculated based on the mass and relative abundance of each isotope.
When we consider an element like Chlorine, we see that it has two main isotopes with mass numbers 35 (nearly 75% abundance) and 37 (about 25% abundance), which determine its average atomic mass. For Rhenium, the average atomic mass reflects the ratio and masses of its isotopes. In calculating average atomic masses, it's important to recognize the weighted average rather than just a simple arithmetic mean of isotope masses.