Iridium metal, Ir crystallizes as a fcc structure. The edge length of the unit cell was found to be 383,9 pm. The density of Ir is 22.42 g/cm3. Calculate the mass of an Iridium atom using the Avogadro number.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.

The answer to your q it is d