Question about electronic configurations of transition metals

[jikuty]

Hi.

Just a quick question about transition metal electronic configurations:

What are the ground-state electronic configurations of the Fe2+ and Fe3+ ions respectively?​

a) [Ar]3d6 and [Ar]3d5
b) [Ar]3d6 and [Ar]3d4 4s1
c) [Ar]3d5 4s1 and[Ar]3d5
d) [Ar]3d4 4s2 and [Ar]3d3 4s2
The correct answer was A. I thought that C would be correct because of the stability of half full subshells. I figured that in Fe2+, the second electron would be removed from the 4s orbital instead of the 3d orbital in order to preserve the half-full 3d subshell. Can anyone clarify?

Thanks.

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[Cjc555777]

Elemental iron is [Ar] 3d6 4s2. There is no electrons in the 4s subshell because it is the outer shell and so the 2 electrons (to give the +2 charge) are removed form there first. Therefore it has no unpaired electrons, as there are 18 (an even number) in the 3rd (outer) shell. It is a little confusing. Also, 4s is a higher energy state. At first glance I would have gone with the same thing, but it's all about energy states

 

[ahs4n]

No, wouldn't 3d(total =5) is still higher than 4s(total =4)

But, otherwise that logic seems right since 4s is still part of the valence electrons.

 

[Cjc555777]

I'm not sure, that's what i thought, the problem was a little confusing. But when I looked it up that was the most common explanation I found.

 

[Fifty 3rds]

where did you get this question? That answer seems incorrect

 

[jikuty]

The question came from one of Kaplan's tests, it's probably also found in the QBank somewhere.

Also, I can see where they got their answer for Fe2+. In the Kaplan material (Physical Sciences Review Notes), it says that for transition metals, electrons are removed from the s orbital before the d orbital. They claim that it's an exception to the (n+l) rule for determining where to remove electrons. I remember that there are a lot of exceptions in the transition metals from 1st year chemistry and that they often result from the stability of half filled subshells, which is why I thought C would be correct.

 

[sv3]

I could be wrong here but this is my understanding of what i found:

For natural state atoms (and not ions), it turns out at least from what I can tell the half/full shell filled rule applies to: chromium, copper, molybdenum, silver, and gold.

Ok, now for ions. Transition metals have to lose their S electrons first no matter what the half/full shell implication may be. So if your gonna touch a D electron, there better not be any S valence electrons sitting there.

I thought the half/full valence shell thing applied to more than just the five elements i listed above but that is what is in TPR. I guess if a question asked about a silver ion, i would take away S electrons first though and follow the rule for ions. I would be tempted to prioritize the half shell thing but it seems losing electrons from the S shell first is steadfast.

God.....another layer of complexity into something i thought was very very simple.

sv3

 

[jikuty]

Thanks a lot for the clarification. Your explanation makes sense and is consistent with the example problems I've seen. I guess it turns out you have to differentiate between neutral atoms and ions for transition elements.

This is also consistent with the Kaplan method of determining the electronic configuration of ions. They state that electrons are removed from the "s" orbital before the "d" orbitals for transition cations (and they don't mention any exceptions).

 

[sv3]

Thanks a lot for the clarification. Your explanation makes sense and is consistent with the example problems I've seen. I guess it turns out you have to differentiate between neutral atoms and ions for transition elements.

This is also consistent with the Kaplan method of determining the electronic configuration of ions. They state that electrons are removed from the "s" orbital before the "d" orbitals for transition cations (and they don't mention any exceptions).

ya well im glad i looked into it as I wasn't aware there were only 5 transition metals that when neutral, try to acquire the half of full shell. I thought it was all of them!!! So you have to remember these 5 I would presume.

thanks for the feedback. Questions like this should be easy....but just goes to show nothing is for free on this bloody beast.

sv3