To calculate the number of atoms in a sample of iron, you can use Avogadro's number, which is approximately 6.022 x 10^23 atoms per mole. First, find the molar mass of iron, which is 55.845 g/mol. Then, divide the mass of the sample (36.0 g) by the molar mass of iron to get the number of moles. Finally, multiply the number of moles by Avogadro's number to find the number of atoms.