What denotes the average of mass numbers of all the isotopes of an element?

The average of mass numbers of all the isotopes of an element is referred to as atomic weight. Atomic weight takes into account the relative abundances of each isotope, which contributes to the average value. This average is expressed in atomic mass units (amu) and represents the weighted average of the isotopes based on how prevalent each is in nature.

It's important to differentiate this from mass number, which refers specifically to the total number of protons and neutrons in a single isotope of an element. The atomic number, on the other hand, indicates the number of protons in the nucleus and defines the element itself, while the radioactive index is not a standard term used in the context of atomic measurements. Thus, atomic weight is the correct term that accurately reflects the average mass of all isotopes of an element.