Question

State Aufbau principle.

Answer 1

"In the ground state of the atoms, the orbitals are filled with electrons in order of increasing energies."

Electrons enters the orbitals according to following $(n+l)$ rules.

a) Orbitals are filled in order of increasing value of $(n+l)$.

Thus, 4s orbital is filled before 3d orbital. For 4s orbital, $(n+l)=4+0=4$. For 3d orbital, $(n+l)=3+2=5$.

b) If two orbitals have same value of $(n+l)$ then the orbital with lower value of n will be filled first.

Thus, 3d orbital is filled before 4p orbital. For 3d orbital, $(n+l)=3+2=5$. For 4p orbital, $(n+l)=4+1=5$. 3d orbital has lower value of n than 4p orbital.