Textbook Problem 1.E.13 - Filling up d-orbital

[Joanne Sarsam 2F]

Hello,

I'm a bit confused on writing the electron configuration with d-block. For part a, when it asks for electron configuration of Ag, the I write . However, the textbook writes it as .
Why do they leave one electron out of the d orbital and instead add 2 to the s orbital? Thank you so much!

[Sophia Zhao]

Technically, the energy required for the electrons to fill the 3d/4d orbital is greater than the 4s/5s orbitals respectively. As seen in the periodic table, the 4s comes before the 3d, and the 5s comes before the 4d. Unless its 4d^5 or 4d^10, typically the 5s orbital fills up before the 4d orbital. The notation lists the 4d before the 5s sequentially, but at 4d^95 5s^2, the orbital for 5s filled before 4d is filled.