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)