Why does 3d come before 4s?

[alexfwang3g]

On the periodic table, 4s comes before 3d so shouldn't it be put before 3d in the electron config?

[Sarah Zhari 1D]

When the 3d orbitals contain electrons, the 3d orbitals have a lower energy than the 4s orbitals. This is caused by electron-electron repulsions, which lower the energy.

[sarahsalama2E]

That is correct, it is also important to note that when 4s begins to fill with electrons, 4s has higher energy than 3d. So, we right 3d before 4s. Also this is for ease of removing an electron when we are forming anions.