Answer
The atomic weight of the element is equal $85.4678amu$, which corresponds to the atomic weight of $rubidium, Rb$.
Work Step by Step
Considering the fractional abundance of the 1+ charged ions of this element ($X$), we can obtain its atomic weight ($Ar(X)$) by following:
$Ar(X)=0.7215\times84.9117amu+0.2785\times86.9085amu=85.4678amu$
Using the Periodic table of elements, we can observe that this atomic weight corresponds to $rubidium$, whose symbol is $Rb$.
