The d-block in electron configurations

[Peiliang Zhou 3K]

Since the number before the letter in an electron configuration refers to the row or period, why do electron configurations for the d-block start with 3d, even though the d-block begins on the 4th period? Is it due to the nature and/or size of the electron sub-shells? Or is it due to differences in energy?

[Eduardogonzalez1G]

Yes. There is a big energy difference between the 3d and 4d shells. When you're doing the electron configuration that includes the d block, you will start with 3d even if youre in the 4th period bc the 3d shell must be filled first