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I'm studying some metallurgy things and interested in iron oxides.

The simple 2-D representation of a mineral like hematite shows $\ce{O=Fe-O-Fe=O}$.

Fe2O3 linear

But the crystal structure seems to show each oxygen making 4 bonds. Crystalline structure from Virtual museum of molecules and minerals[1]

Fe2O3 crystalline

So whats the deal here?

Reference:

[1]: Hematite – virtual museum of molecules and minerals
https://virtual-museum.soils.wisc.edu/display/hematite/
(accessed Jun 16, 2021).

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human

This is a human.

Well, sort of.

All right, call it a simplified representation. But at least it shows the arms/legs ratio accurately, you can't deny that.


Now that was about as good as the molecular formula of $\ce{Fe2O3}$ (a compound which has no molecules in the first place):

Fe2O3

As for the crystal structure image, that's what $\ce{Fe2O3}$ actually is. The sticks are drawn where the two atoms touch. Call them bonds or otherwise, the Nature does not care.

When you learn about the coordination compounds, the idea of atoms making far more bonds than their formal valence allows won't look that weird.

So it goes.

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    $\begingroup$ +1 The first image is so cursed ;/ $\endgroup$ Jun 17 at 8:54
  • $\begingroup$ thanks for the answer. As im interested in the redox reactions taking place in these crystals with CO and H2 reduction, I'm really curious about the electrons being shared. $\endgroup$
    – dlight
    Jun 17 at 9:12
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    $\begingroup$ Reactions don't take place in the crystals. On the surface, maybe. $\endgroup$ Jun 17 at 9:31
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    $\begingroup$ @NilayGhosh Still not as cursed as this $\ce{O=Fe-O-Fe=O}$ abomination. $\endgroup$
    – andselisk
    Jun 17 at 9:35
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There is no relation between the formula of a substance and the number of neighbors any given atom has in its vicinity. For example, each sodium cation $\ce{Na+}$ is surrounded by $6$ $\ce{Cl-}$ ions in the crystal of salt, although its formula is $\ce{NaCl}$. Each sodium atom has $6$ neighbors, but it stays at oxidation number $+1$.

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  • $\begingroup$ So the crystal structure is not showing bonds, what is it showing? $\endgroup$
    – dlight
    Jun 16 at 19:06
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    $\begingroup$ It is showing the local nearest neighbours for each atom. Don't think of ionic bonding as occurring pairwise. As coulomb interactions are very long ranged an ion sits in the electric potential due to the whole of the lattice, and so is interacting with every other ion in the lattice. A bonds to B in such a situation is just not a good model. $\endgroup$
    – Ian Bush
    Jun 16 at 19:18

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