Answer
Please see the work below.
Work Step by Step
We know that the number of sodium atoms in a $1.00mg$ crystal of sodium is
$1\times 10^{-3}g \space Na\times \frac{1 mol\space Na}{22.99g\space Na}\times\frac{6.02\times 10^{23}Na\space atoms}{1 \space mol}=2.618\times 10^{19} Na \space atoms$
