And again...chemistry...?

Well, in the fourth period, the 3d orbitals are introduced. However, orbitals fill the (n - 1)d orbital after the ns orbital. The reason for this is the s orbtials are lower energy. Actually, the ns orbitals are further away from the nucleus than (n - 1)d orbitals, but the ns orbitals penetrate more frequently and spend more time closer to the nucleus. The best way of knowing what order the electrons occupy orbitals is to read the periodic table from left-to-right. Notice the 3d orbitals come after the 4s orbitals.

Also, I don't believe the reason is totally understood. But it seems that having either one or two electrons in all five of the d orbitals is more stable than, say, having an electron configuration of 4s2 3d4 or 4s2 3d9. Instead, what is observed is an electron from the s orbital is transferred to the d orbital. So istead you would have: 4s1 3d5 or 4s1 3d10. This is rather abnormal.