Answer:
1. Isotope with mass number 39.
2. 39.02g/mol
Explanation:
1. The most common isotope is the isotope in the higher proportion, that is:
Isotope with mass number 39.
2. The average atomic mass is the sum of the masses times their abundance. For the element X:
Average atomic mass:
38*0.0967 + 39*0.7868 + 40*0.1134 + 41*0.0031
= 39.02g/mol
