There are four naturally occurring isotopes of strontium: ${ }_{38}^{84} \mathrm{Sr}$,${ }_{38}^{86} \mathrm{Sr},{ }_{38}^{87} \mathrm{Sr},$ and ${ }_{38}^{88} \mathrm{Sr} .(4.4,4.5)$
a. How many protons, neutrons, and electrons are in Sr- $87 ?$
b. What is the most abundant isotope in a strontium sample?
c. How many neutrons are in Sr-84?
d. Why don't any of the isotopes of strontium have the atomic mass of 87.62 amu listed on the periodic table?