Fe(no3)2 not sure how to get the oxidation numbers of all elements

The oxidation number of oxgyen is -2. In the compound, the total charge of oxygen is -12. Fe has a charge of +2. To make the sum equal to zero, N has to have a charge of +5. -12+2+2x=0; x= +5. 

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.