How do the isotopes hydrogen-1 and hydrogen-2 differ? Hydrogen-2 has one proton; hydrogen-1 has none. Hydrogen-2 has two protons; hydrogen-1 has one. Hydrogen-2 has one neutron; hydrogen-1 has none. Hydrogen-2 has one more electron than hydrogen-1.

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.