Rubidium is a soft, silvery-white metal that has two common isotopes, #""^85Rb# and #""^87Rb#. If the abundance of #""^85Rb# is 72.2% and the abundance of #""^87Rb# is 27.8%, what is the average atomic mass of rubidium?
More specifically, the average atomic mass of an element is calculated by taking into account the atomic masses of its stable isotopes and their decimal abundance, which is imply the percent abundance divided by
#color(blue)("avg. atomic mass" = sum_i ("isotope"_i xx "abundance"_i))#
So, you know that rubidium has two stable isotopes,
When the problem doesn't provide the atomic masses of the stable isotopes, you can use their mass numbers. As you know, an isotope's mass number tells you how many protons and neutrons it contains in its nucleus.
In this case,
The average atomic mass of rubidium will thus be
#"avg. atomic mass" = "85 u" xx 0.722 + "87 u" xx 0.278#
#"avg. atomic mass " = " 85.556 u"#
Rounded to three sig figs, the number of sig figs you have for the percent abundances of the two isotopes, the answer will be
#"avg. atomic mass " = color(green)(" 85.6 u")#