Answer
Please see the work below.
Work Step by Step
We know that
a.$\frac{7.92g\space As}{1mol\space As}\times \frac{1mol\space As}{6.022\times 10^{23}As\space atoms}=1.244\times 10^{-22}\frac{g}{As\space atom}$
b. $\frac{58.69g\space Ni}{1mol}\times \frac{mol\space Ni}{6.022\times 10^{23}Ni\space atoms}=9.746\times 10^{-23}\space \frac{g}{Ni\space atom}$
