The number of moles is equal to the number of particles of a substance in a given sample divided by the Avogadro's number.

Moles = Number of sodium atoms/ Avogadro's number

Sodium atoms = 1.56 x 10^21

Avogadro number = 6.022 x 10^23

Moles = 1.56 x 10^21 / 6.022 x 10^23

Moles = 0.26 x 10^-2

So, there are 0.26 x 10^-2 moles of sodium in 1.56 x 10^23 atom.

Moles = Number of sodium atoms/ Avogadro's number

Sodium atoms = 1.56 x 10^21

Avogadro number = 6.022 x 10^23

Moles = 1.56 x 10^21 / 6.022 x 10^23

Moles = 0.26 x 10^-2

So, there are 0.26 x 10^-2 moles of sodium in 1.56 x 10^23 atom.