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.

Why do 3d comes before 4s?

We say that the 4s orbitals have a lower energy than the 3d, and so the 4s orbitals are filled first. ... The electrons lost first will come from the highest energy level, furthest from the influence of the nucleus. So the 4s orbital must have a higher energy than the 3d orbitals.

Why 3d has more energy than 4s?

Ans: Once 3d orbitals are occupied by electrons, like in the case of transition elements, because they are closer to the nucleus, they will repel the 4s electrons further away from the nucleus and cause it to have higher energy level.