Origin of the lab: I believe this lab goes all the way back to my first experience being an Assistant Instructor at Indiana University with Jim Brophy, though part two was added to my collection of activities a year later by Enrique Merino (also IU). I've modified, tweaked, and just plain added & subtracted from this lab for 10 years (!). Its not perfect, but here's where I'm at.
Mineral models come in a few different types, but the most common is the "ball and stick" model, where the ball = atoms and sticks = bonds. Though they distort the size relationships between cations & anions, the models are useful when talking to students about coordination and trying to grasp what the structure of the different mineral groups look like. Ball & stick models are delicate, hard to replace, and prone to collecting dust, so they should be handled with care.
This lab will work for whatever ball and stick models your department happens to own, though you may find that some of the concepts are not well demonstrated by the models you have on hand. I'll add a list at the end of models I believe are "most" useful for this lab & why.
The last portion of this lab deals with a different type of coordination model and is represented by equal-sized balls of some sort (I'm using styrofoam this semester, but ping-pong, tennis, etc. all work as long as you can somehow stick them together with glue or toothpicks). This portion deals with 12-fold coordination.
If you need a copy of the figures I reference, please just post a comment over on my blog.
· For each model in the lab you should make a chart that includes:
o Formula (either your textbook or DHZ; when there are multiple possibilities ASK)
o Color key (e.g. O2- = red, Fe2+ = blue, Al3+ = silver)
o For each cation in that structure answer the following questions:
§ What anions is it bonded to?
§ What is the coordination number?
§ What is the shape of the coordination polyhedra? (e.g. cubic, octahedral)
§ What is the atomic radius of the cation? (K&D table 3.8)
§ Is the polyhedra regular or irregular? (look at the angles between all of the bonds; if equal = regular)
§ What is the electronegativity of the cation? (K&D Fig. 3.20)
§ Using K&D Fig. 3.21, what is the character of the bond?
· In Excel, make a plot with CN on the vertical axis and cation radius on the horizontal axis
o Label the axes
o Make sure each point on the graph is labeled with the name of the cation & the mineral it is in
o Plot the points from smallest to largest cation radius as you move from left to right
o Draw a line between each break in CN (freehand)
· With your plot in front of you:
o What is the relationship between cation radius and coordination number?
o Which cations are most likely to substitute for each other?
o Does the percent of ionic character play a role in where certain cations plot?
o Does percent of ionic character impact the regular vs. irregular character of a polyhedra?
o Does cation radius have an influence on regular vs. irregular?
· Styrofoam balls:
o Make four identical layers using 9 balls per layers so they look like K&D Fig. 4.1a
o How many different ways can you stack the four layers? (Hint: look at the text associated with K&D Fig. 4.1a)
o Choose a ball in the middle of the stacked layers – what is its coordination number?
§ Repeat for each way you discovered to stack the layers.
§ Does the number vary?
o Look at the void space and try to stick a pencil / other long skinny instrument down through the voids
§ Does the organization of the void spaces vary or is it uniform for the different ways you discovered to stack the layers?
§ Are the voids a uniform size or do they vary?
Last revision: 14. February 2011