Final answer:
The element with the highest average atomic mass is uranium (element 92) with an average atomic mass of approximately 238 u, as it has the heaviest stable isotopes that occur naturally.
Step-by-step explanation:
The element with the greatest average atomic mass is determined using the weighted average mass of all its naturally occurring isotopes. Technetium (element 43), promethium (element 61), and elements with atomic number greater than or equal to polonium (84) have unstable, radioactive isotopes and thus do not have a standard average atomic weight. Instead, they have an approximate atomic mass listed in square brackets, corresponding to the most stable isotope's mass number. To find the element with the greatest average atomic mass, we look at the heaviest naturally occurring elements, which can be found towards the bottom of the periodic table. As of the knowledge cutoff date in early 2023, the element with the highest atomic number and stable isotopes available in nature is uranium (element 92), with an average atomic mass of approximately 238 u.