1. QUESTION:
What is the electron configuration of Fe3+? A. [Ar] 4s2 3d6 B. [Ar] 4s2 3d3
C. [Ar] 4s1 3d4
D. [Ar] 3d5
SCRIPT:
What is the electron configuration of ferric ion?
A. [Ar] 4s2 3d6 B. [Ar] 4s2 3d3
C. [Ar] 4s1 3d4
D. [Ar] 3d5
PAUSE
Let’s look up iron in the periodic table.
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2. Fe: 26 electrons [Ar] = 1s2 2s2 2p6 3s2 3p6, 18 electrons 4s 3d
SCRIPT, slide 2
Here’s iron… it’s atomic number is 26. Therefore it has 26 electrons.
CLICK
Iron is in the fourth period.
Here’s period 1…
Period 2
Period 3
Period 4…
The noble gas the immediately preceding iron is the Argon.
So the electron configuraiton of iron has an argon core.
The argon core, 1s2 2s2 2p6 3s2 3p6… accounts for 18 electrons
26 minus 18 is 8
We have eight more electrons to assign.
Two of the eight electrons go into the 4s subshell…
And the last six go into the 3d subshell…
One
Two
CLICK Three
Four
Five
CLICK Six
Therefore… the electron configuration of an iron ATOM is … argon core, 4s2, 3d6
Now let’s go back to the question…
[Ar] 4s2 3d6

3. QUESTION:
What is the electron configuration of Fe3+? A. [Ar] 4s2 3d6 B. [Ar] 4s2 3d3
C. [Ar] 4s1 3d4
D. [Ar] 3d5
When transition metals ionize, the valence electrons are the first to go. Ground state electron configurations:
Fe [Ar] 4s2 3d Fe2+ [Ar] 3d6
Fe3+ [Ar] 3d5
SCRIPT: slide 3
We can see that the electron configuration given in choice A is the electron configuration of the iron ATOM.
We’re being asked for the electron configuration of the iron ion with a +3 charge. So choice A is not correct.
A +3 charge means a loss of three electrons. When transition metals lose electrons, the first electrons to go are the valence electrons.
In the case of iron, the valence electrons are the electrons in the fourth shell…
HIGHLIGHT 4s2
Losing the two electrons in the fourth shell gives us an ion with a +2 charge.
HIGHLIGHT Fe2+
We need to remove one more electron to get an ion with a +3 charge.
We take that electron out of the 3d subshell.
That leaves 5 electrons in the 3d subshell.
The correct answer is D.
Note that we’re talking here about the ground state or most stable configurations. Choices B, C, and D are all valid configurations.
Choices B and C are examples of excited state configurations. Unless otherwise specified or obviously implied, when we talk about electron configurations, we mean the ground state.
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4. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08