Final answer:
The average (weighted) mass of element Zz is calculated using the percentage of each isotope and its corresponding atomic mass, resulting in an average atomic mass of 120.9 amu.
Step-by-step explanation:
To calculate the average (weighted) mass of element Zz given that 82% of its atoms have a mass of 120 amu, and 18% have a mass of 125 amu, we use the concept of a weighted average. The calculation is similar to the method used to find the average atomic mass of an element, which considers the relative abundances and masses of its natural isotopes.
The weighted average atomic mass of element Zz is given by:
(0.82 × 120 amu) + (0.18 × 125 amu) = 98.4 amu + 22.5 amu = 120.9 amu.
The average atomic mass of element Zz is therefore 120.9 amu. This is the value that would typically be listed under the symbol for Zz in the periodic table, representing the weighted mass of its naturally occurring isotopes.