3d and 4f

[Lily Cornett 1H]

Hi! I understand when doing electron configurations the d block starts with 3 and f block starts with 4. But why are they one and two smaller, respectively, to the S and P block?

[906118466]

The d block is offset by 1 because it starts in the fourth row of the periodic table with Sc. The 3d orbitals have higher energy than the 4s orbitals so the 3d orbitals are only filled after the 4s orbitals are full. The f block is offset by 2 because it begins in the sixth row of the periodic table and the 4f orbitals only start filling after the 6s and 5d orbitals are full.