2.5 million atoms of a particular element have a mass of 8.33 x 10-16 grams. what is this element

Find the mass of one atom by:

8.33 x 10^{-16} / 2.5 x 10^{6}  = 3.32 x 10^{-22} g

Convert it into amu.
1 g = 6.022 x 10^{23} amu
so, 3.32 x 10^{-22}  x  6.022 x 10^{23}  =  200.6 amu

Now look at the periodic table and search for 200.6 amu element.
The element is Mercury (Hg)

Anions are negative charged ions