Highest energy electron

[lovelearning]

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I was studying my PR Gen Chem Review book and came across this freestanding practice question at the end of Ch. 3:

Which of the following possesses the highest energy electron?
A) Br^-
B) Ca^2+
C) Cr^+
D) As

The answer listed is D, and the book's explanation is, "Electron energy level is determined by the 1st 2 quantum numbers. Given Br- = [Kr], Ca2+ = [Ar], Cr+ = [Ar]3d^5, and As = [Ar]4s^23d^104p^3, arsenic contains electrons in the highest energy subshell, 4p."

That said, Br- should also have electrons in the 4p subshell. I'm unsure why Br- doesn't have electrons that are just as high energy. Does the fact that Br- has a full 4p subshell make all of its 4p electrons a slightly lower energy? Thanks in advance.

 

[krogers21]

I was studying my PR Gen Chem Review book and came across this freestanding practice question at the end of Ch. 3:

Which of the following possesses the highest energy electron?
A) Br^-
B) Ca^2+
C) Cr^+
D) As

The answer listed is D, and the book's explanation is, "Electron energy level is determined by the 1st 2 quantum numbers. Given Br- = [Kr], Ca2+ = [Ar], Cr+ = [Ar]3d^5, and As = [Ar]4s^23d^104p^3, arsenic contains electrons in the highest energy subshell, 4p."

That said, Br- should also have electrons in the 4p subshell. I'm unsure why Br- doesn't have electrons that are just as high energy. Does the fact that Br- has a full 4p subshell make all of its 4p electrons a slightly lower energy? Thanks in advance.

I think you're on the right track here. The more stable something is, the less energy it has. That being said, Br- is more stable since the 4p subshell is filled whereas As has only half the subshell filled - so not quite as stable although it is more stable than say Ge. However, I think the answer given is quite poor.

 

[lovelearning]

Okay, thanks for the help! I guess the explanation just should've differentiated between A and D better...