Why in building of the atoms the filling of 4s orbital takes place before 3d orbital ?

Explanation

According to Bohr.s Bury.s rule, energy of different arbitals can be calculated by (n+l) values. According to Aufbau principle, orbitals of lower energy are filled first then the other orbitals.
For `4s : (n+l)=(4 + 0) = 4`
For `3d : (n+l) = (3 + 2) = 5`
That is why 4s orbital is filled before 3d.