1. Avogadro's Number

Avogadro's number tells us the number of atoms or molecules in one mole of a substance:

* 1 mole = 6.022 x 10^23 atoms (or molecules)

2. Setting up the Calculation

We can use a proportion to find the number of moles:

(1.56 x 10^21 sodium atoms) / (x moles) = (6.022 x 10^23 atoms) / (1 mole)

3. Solving for x

* Cross-multiply: (1.56 x 10^21 atoms) * (1 mole) = (6.022 x 10^23 atoms) * (x moles)

* Simplify: 1.56 x 10^21 = (6.022 x 10^23) * x

* Isolate x: x = (1.56 x 10^21) / (6.022 x 10^23)

* Calculate: x ≈ 0.00259 moles

Answer: 1.56 x 10^21 sodium atoms correspond to approximately 0.00259 moles of sodium.