Placement of the 3d orbital

[Nathan Mariano 2G]

Why is the 3d orbital placed before the 4s orbital? When viewing ptable.com (Dynamic Periodic Table), the 3d orbital is placed after the 4s orbital.

[Ariel Cheng 2I]

The 4s orbital comes before the 3d orbital because the 4s shell actually has a lower energy level than the 3d orbital. However, if the 4s orbital is filled (2 electrons), then its energy level rises to higher than the 3d orbital which is why we write it after the 3d (exceptions: Chromium and Copper).