Question: "Which of the following possesses the highest energy electron?"
A.) Br-
B.) Ca(2+)
C.) Cr+
D.) As
Answer is (D), but I wanted to clarify why (A) is wrong. Br- is isoelectronic with [Kr], which has electrons in the 4p subshell like As does, but are the electrons in As higher energy because [Kr] is a noble gas w/ a full 4p subshell, thus lower energy electrons?
The book explanation (TPR) didn't seem to address this
Hmm. I picked A too, because first instinct, I was thinking of energy levels and being that the valence electrons in Br would be in the highest energy level, I sorta assumed they would have the highest energy electron. But then I realized the first three choices are ions, not ground-state elements, so that's not a valid comparison.
Choices A, B, and C are all somewhat stabilized by the loss or gain of an electron(s). Bromine, a nonmetal, wants to gain an electron to become like a noble gas, and by doing so it becomes stable. Calcium, a metal, wants to lose to electrons to become like nobel gas Argon. It too is stable. And finally, to understand why Chromium is stable you need to consider the electron configuration: Cr: [Ar]4s13d5 It's one of those odd-ball exceptions. Like calcium, it too is a metal and therefore tends to lose an electron and attains a noble gas configuration -- Cr: [Ar]3d5 .. well sort of. It has half-filled d-shell stability, which is a very favorable.
Now choice D, in my opinion is arguable because some would argue that having three electrons in the p-orbitals would make it somewhat stable. But I suppose its more favorable and therefore more stable to become like a noble gas and so naturally, it would want to gain three electrons. In comparison to the other answer choices, this would be the clear choice I suppose.
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I think a more simple way to answer this question is by realizing Bromium, Calcium, and Chromium ions all have noble gas configurations (ignoring d-shell electrons in Chromium). A noble gas is less likely to lose an electron (think in terms of the ionization energy trend). Arsenic on the other hand wants to become like a noble gas, so it would need to gain three electrons to become stabilized. In it's present state, it's very reactive making the electrons very unstable in comparison to the first three choices.
