Answer
Please see the work below.
Work Step by Step
First of all, we find the number of moles for 2 atoms of Pb and then convert it into mass
$2\space atoms \space Pb\times\frac{1mol}{6.022\times 10^{23}atoms}=3.321\times 10^{-24}mol\space Pb$
$3.321\times 10^{-24} mol \space Pb\times \frac{207.2g\space Pb}{1mol\space Pb}=6.881\times 10^{-22}g\space Pb$
Now we convert $5.1\times 10^{-23}$ mol of He to mass
$5.1\times 10^{-23}mol\space He \times \frac{4.003g\space He}{1mol\space He}=2.0\times 10^{-22}g\space He$
Thus 2 atoms of lead has greater mass than $5.1\times 10^{-23} mol$ of He.
