When a bimetallic bar made of copper and iron strips is heated, the bar bends toward the iron strip. the reason for this is?

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.

Sorry i dont understand the question can you rephase it?