In general chemistry pertaining specifically to elements, isotopes belong to the same element. They have the same atomic number or number of protons, but differ in mass number which is the number of protons and neutrons in the nucleus. The number of isotopes of an element depending on what's existing in nature. For Strontium, it has 4 isotopes. The average atomic mass that is listed in the periodic table is taken from the masses of each of its isotopes. To calculate for this, you multiply each mass of the isotope with its percentage of abundance, then add them up altogether.
Average atomic mass of Strontium = 83.9134(0.5%) + 85.9094(9.9%) , 86.9089(7.0%) + 87.9056(82.6%)
Average atomic mass of Strontium = 87.6182 amu
Therefore, the mass of Strontium is 87.6182 amu.