Answer:
The atomic mass of rubidium is 85.468 u
Step-by-step explanation:
To determine the average atomic mass of an element, we take the weight average of the atomic masses of its naturally occurring isotopes. This means that alll isotopes together are calculated by multiplying the atomic mass  and the percentage of abundance . So we have sum of all the different isotopes ( which is 100 % of abundance)
For naturally occurring rubidium, there are 2 isotopes.
mass of Rb-85 x abundance + mass of Rb-87 x abundance
84.9118 x 0.7217 + 86.9092 x 0.2783 = 85,46767642
On the periodic table we can see for Rubidium 85.468 u which is the average atomic mass of Rubium ( and its 2 naturally occurring isotopes).
