When polonium-210 decays by alpha radiation what isotope is formed?

Alpha radiation or alpha decay is a type of radioactive decay in which the nucleus of the atom emits an alpha particle. Alpha particle has a structure similar to that of helium-4 nucleus.

Therefore:
When polonium-210 decays by alpha radiation, Lead - 206 is formed.

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.