Question

Why some time we write 4s befor 3d and some time after 3d

Answer 1

Electrons usually enter the 4s orbital before the 3d because the 4s is initially lower in energy. However, from Scandium and onwards, the 3d orbital of transition metals actually becomes lower in energy than the 4s, which is why we write 3d before 4s in the configuration.

Answer 2

Answer:

According to the aufbau principle the 4s orbital is lower in energy than the 3d orbital hence, it is filled first. However, when we consider a transition metal complex this does not apply; the 3d orbital is filled before the 4s orbital.