Final answer:
The average atomic mass of an element with three isotopes can be calculated by multiplying each isotopic mass by its relative abundance and summing the results. Isotope X-7.00 is excluded from the calculation because its abundance is 0.00%.
Step-by-step explanation:
To calculate the average atomic mass (AMU) for an element with multiple isotopes, one must use the given abundances and isotopic masses. The calculation is the weighted mean of the isotopic masses, where each isotope's mass is multiplied by its relative abundance (expressed as a decimal).
- For isotope X-4.00 with an abundance of 87.46%, the contribution to the AMU is 4.00 amu × 0.8746.
- For isotope X-5.00 with an abundance of 11.57%, the contribution to the AMU is 5.00 amu × 0.1157.
- For isotope X-6.00 with an abundance of 0.97%, the contribution to the AMU is 6.00 amu × 0.0097.
The weighted average is found by summing these contributions:
(4.00 amu × 0.8746) + (5.00 amu × 0.1157) + (6.00 amu × 0.0097) = the average atomic mass of the element.
Note that isotope X-7.00 is not considered because its abundance is 0.00%.