The astronaut then measures the abundance of strontium on the new planet, obtaining the following results:

isotope abundance
(%) mass
(amu)
86sr 9.46 85.91

87sr

7.00 86.91
88sr 83.54 87.91

what is the atomic mass of strontium for this planet?

Thank you for posting your question here at . I hope the answer will help you. Feel free to ask more questions.
Based on the percent abundances, give an estimate of the atomic mass. 

The atomic mass will be between87 and 88.86 and 87.amuamu

The answer should be 87.65amu

Both models are effective because they show the valence electrons, and these valence electrons indicate the reactivity of an element.