solids - crystal structures

[robert zellmer]

This is sections 11.7 and 11.8 in the 10th and 11th editions and
this same material from the 11th edition on Carmen for those using
the 12th and 13th editions.  Remember, no matter which edition
you're using we are not using chapter 12.

Last week I covered the primitive(simple), body-centered and face-centered
cubic structures.  I gave a lot of info about these (# particles/uc,
how the particles touch, relationship between edge-length and radius,
coordination number, % empty space, type of holes the particles
are in, etc.).  I also covered close-packing of spheres and we saw
a non-cubic type of packing (Hexagonal close-packing) and learned
the cubic-close packed structure is the same as a fcc structure.
All of this discussion was for identical particles in the u.c. (i.e. same
size, like all Fe atoms, all CO2 molecules, etc.)

This was all covered in section 11.7 in the 10th and 11th editions
Some of this is summarized in the following link (found at the "Notes"
link on my web page):

*Chapter 11/12 - Unit Cell Table for Identical Sized Particles* 
<http://chemistry.osu.edu/%7Erzellmer/chem1250/notes/unit_cell_table.pdf>

Also, in the 10th and 11th editions the structure of NaCl is discussed in
section 11.7 and CsCl, ZnS and CaF2 are discussed in section 11.8.
In the 12th edition ionic unit cells are discussed in section 12.5, but 
again
it is best to use what's on Carmen.

NaCl is described as a face-centered cubic unit cell.  The problem is
it doesn't look like the fcc unit cell described for a substance in which
all particles are identical.  I explained why it is called a fcc 
structure and
how it differs from the fcc unit cell for identical sized particles in 
lecture.
For NaCl understand the unit cells and related information given in class,
notes and the textbook for NaCl.  This includes its structure, placement
of the ions, how anions and cations touch (along an edge), relationship
between edge length and radii of the ions, number of nearest neighbors
and what they are, for both the cation and anion, type of "hole" each ion
"sits" in, unit cell calculations, etc.

For CsCl, ZnS and CaF2 you should know where the atoms are in the u.c.
In doing unit cell calc. for ionic structures you must know how many
formula units there are in a unit cell (e.g. NaCl, ZnS and CaF2 all have
4 f.u. per unit cell, CsCl has 1 fu/uc).  For CsCl, ZnS and CaF2 you only
need to know the # fu/uc.  I've included a link to a table with all the
information for these 4 ionic structures.  You should  know what's listed
for NaCl.  For the other 3 ionic crystal structures just know the placement
of the ions and the #fu/uc.  You can find what you need for these ionic
structures at the following link (found at the "Notes" link on my web page):

*Chapter 11/12 - Unit Cell Table for Ionic Structures (know stuff in 
red)* 
<http://chemistry.osu.edu/%7Erzellmer/chem1250/notes/ionic_unit_cell_table_filled.pdf>

Furthermore, I would highly suggest using dimensional analysis to
do unit-cell calculations rather than the way shown in the book.  I did
them this way in class and I think in the long run it makes them
easier.  You will find you can do the uc calc. for ionic structures in the
same way as you do for atoms or molecules if you think in terms of
"formula-units" (f.u.) for the ionic compounds.  NaCl is the "formula unit"
for sodium chloride.  You treat it just as if it were a molecule. For
CO2, which is a fcc structure, you have 4 CO2 molecules per u.c. and
for NaCl you have 4 NaCl f.u. per u.c.  Thus, you can do a density
calculation the same way for CO2 and NaCl as you would for something
like the Ag example I did in class (using the MW for CO2 or the formula wt.
for NaCl and the number of particles/uc in the same way you would use
the atomic wt. and # atoms/uc for an atom like Ag).

For unit cell calculations you should be able determine the following:
density, volume (edge length and particle radius), AW (MW, FW),
unit cell type (sc, bcc, fcc) and Avogadro's Number.
This includes ionic structures.

Don't forget about the following web site where you can look at these
structures in 3-D and rotate them around.  Click the "All atoms" button.
Ask for more than 1 unit cell when trying to see the nearest neighbors
(coord. #).  Also, for the ionic compounds it's much easier to see the
nearest neighbors if you click on "Connect Atoms" toward the bottom
of the window (you sometimes have to click it twice).

http://undergrad-ed.chemistry.ohio-state.edu/xtal/index.html

Also, look at the links I have for ch 11/12 ("Notes" link).  I have the 
example
in the notes for determining Avogadro's Number (which was not done in
lecture), the table given in the notes with the info about the unit 
cells for
identical particles (including packing) and a table for the 4 ionic 
structures
in the notes (with the info you should know in red).  The links for these
last two things were given above.

Also, look at the last page of the chapter 11 (12) notes.  You will find 
a table
summarizing the 4 types of solids and their properties.

I hope this makes sense.

Dr. Zellmer
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