Convert atoms to moles, and then moles to grams.
5.50 atoms * [tex]\frac{1 mol Hg}{6.02E23 atoms}[/tex] * [tex]\frac{200.59g Hg}{1 mol Hg}[/tex] = 1.83 * [tex]10^{-21}[/tex] grams
Check if the answer makes sense:
5.5 atoms is an extremely small fraction of a mole, therefore the total grams will be an extremely small fraction of the molar mass of Hg.
