When 21.44 moles of Si react with 17.62 moles of N2

3 Si + 2 ⟶ Si3N4

How much of the excess reactant remains after the reaction?

They are like other investigations? say there was a murder u have to do blood test and finger prints which run through a lab. 

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.