Final answer:
To estimate the average atomic mass of element J, sum the products of each isotope's mass number and its relative abundance, resulting in an estimated average atomic mass of 18.5 amu.
Step-by-step explanation:
To estimate the average atomic mass of the hypothetical element J with an atomic number of 8 and two isotopes (one with 9 neutrons and the other with 12 neutrons), we can use the following method:
- Determine the mass numbers of the isotopes by adding the number of protons (which is 8 for element J) to the number of neutrons in each isotope. This results in mass numbers of 17 (8 protons + 9 neutrons) and 20 (8 protons + 12 neutrons), respectively.
- Since the isotopes are equally abundant, their abundance fractions are both 0.5 (or 50%).
- Calculate the weighted average mass by multiplying the mass number of each isotope by its abundance and adding the results. Average atomic mass = (mass of isotope 1 × abundance of isotope 1) + (mass of isotope 2 × abundance of isotope 2).
Using the information above:
Average atomic mass = (17 amu × 0.5) + (20 amu × 0.5) = 8.5 amu + 10 amu = 18.5 amu.
Therefore, the estimated average atomic mass of element J is 18.5 amu.