The electron enters into 4 s orbital after filling 3 p orbital but not into 3d . Explain the reason .

Explanation

Based on (n + l) values energy value of 3d orbitals is 3 + 2 = 5 , energy value of 4s orbitals is 4 + 0 = 4
The energy level of 4s orbital is less than the 3d orbital according to the Aufbau principle electrons enters into lower energy orbital first .
Thats why electrons into 4s after filling 3p , but not into 3d .