To determine the exact atomic mass of the mystery element, we need to calculate the weighted average of the isotopes' masses, considering their percent abundances.
Step 1: Convert the percent abundances to decimal form: 28.27% abundance = 0.2827 39% abundance = 0.39
Step 2: Multiply each isotope's mass by its respective percent abundance: (144.288 amu) * 0.2827 = 40.8197136 amu (143.66 amu) * 0.39 = 56.0434 amu (149.907 amu) * 1 = 149.907 amu
Step 3: Add the products from Step 2 to obtain the weighted average: 40.8197136 amu + 56.0434 amu + 149.907 amu = 246.77 amu
Therefore, the exact atomic mass of the mystery element is approximately 246.77 atomic mass units (amu).