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Name:  Ms. T
Status: educator
Grade: 9-12
Location:CA
Country: N/A
Date: 10/16/2005


Question:
What is the charge on an Fe3O4 iron ion? If I assume that each oxygen ion has a charge of -2, then it would appear that each iron ion has a charge of 2.66. How can an ionic compound have an ion with a non-whole number charge? Thanks for any clarification here.


Replies:
An excellent question!

Fe3O4 is magnetite, one of the major iron ores found in the earth's crust. It is indeed a binary ionic compound, made of a metal (Fe) and a nonmetal (O).

Iron exists in two different charge states in this compound, +2 (II) and +3 (III). It is sometimes called a "mixed oxide." Note that the average charge per Fe atom can be calculated by assuming that two are +3 and one is +2:

(3 + 3 + 2)/(3 Fe atoms) = 8/3 = 2.66...

The Fe (II) ions are located in octahedral interstices within the crystal lattice and Fe (III) ions are located in two different interstices (some tetrahedral, some octahedral). Magnetite is a pretty good conductor, unlike the other iron oxides; some think that's because of "rapid valence oscillation between the Fe sites" (electrons jumping from ion to ion, which does not happen much when all the ions have the same charge)

(Source: Cotton, Wilkinson, Murillo and Bochmann , Advanced Inorganic Chemistry, 6th ed. (John Wiley & Sons, NY, 1999))

Hope this helped,

Dr. Topper


You will notice that 2.66 is a nice, rational fraction: 2 and 2/3. 2+(2/3) = (6/3)+(2/3) = 8/3, which implies that if 3 iron atoms have 8 positive charges, it works out perfectly. The set of ions that does "8 in 3" is: 2:Fe(+3) + 1:Fe(+2). Both +3 and +2 are known comfortable charges for iron to carry. The mineral's formula would be Fe2O3*FeO, which you might consider a mixed compound, but it happens, especially this one. Integer-ratio alloys of two compounds often make a new identifiable compound, whenever they can form a crystal-structure neat enough to have low energy. Every crystal of a hydrated salt is an example of this category.

The two different oxidation states are real, atom by atom, not just some strange semi-quantization of a uniform non-integer oxidation number, so if I looked at the crystal structure of Fe3O4 I would expect to see two spatially different Iron-atom positions in the crystal lattice, probably with differing numbers of Oxygen-atom neighbors or iron-atom 2nd-neighbors. But I would not generalize much beyond that: the Fe2O3 and FeO parts do not need to be identifiably separate molecules or anything like that. Fe3O4 is one compound, with one crystal structure that might appear tight but slightly stagger-patterned.

Jim Swenson


Not all of the 3 iron atoms has the same "formal charge". You have 3 Fe's that have to add up to +8. How about +3 +3 +2 ?

Steve Ross


The scheme used in introductory chemistry of assigning oxidation numbers to various elements should be used as a guide, in the world of real chemistry there are many exceptions, which unfortunately are usually not addressed in introductory chemistry. Fe3O4 is an example. It is a complex oxide with varying amounts of Fe(+3) and Fe(+2) and also contains other inclusions and/or holes. The structure of Fe3O4 is given at:

http://www.webelements.com/webelements/compounds/text/Fe/Fe3O4-1317619.html

There are many compounds for which the usual assignment of oxidation number fails. Both texts and teachers fail to make those "exceptions" clear to students.

Vince Calder



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