When the redox reaction S2O82- Cr3 SO42- Cr2O72- is balanced in acid solution, the coefficients in front of these four species are

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.

Its a hope this