14 May 2016

Crystal Field Theory and Gems--Guided Inquiry

In-Class Activity

Submitted by Adam R. Johnson, Harvey Mudd College
Categories
Description: 

The colors of transition metal compounds are highly variable. Aqueous solutions of nickel are green, of copper are blue, and of vanadium can range from yellow to blue to green to violet. What is the origin of these colors? A simple geometrical model known as crystal field theory can be used to differentiate the 5 d orbitals in energy. When an electron in a low-lying orbital interacts with visible light, the electron can be promoted to a higher-lying orbital with the absorption of a photon. Our brains perceive this as color. Rubies, dark red, and emeralds, brilliant green, are precious gemstones known since antiquity. What causes the color in these beautiful crystals? Using crystal field theory, we can explain the colors in these gemstones.

Learning Goals: 

1.    Derive the crystal field splitting for d orbitals in an octahedral geometry
2.    Predict the magnitude of d orbital splitting
3.    Relate color, energy, wavelength, and crystal field strength
 

Equipment needs: 

Day 1: none

Day 2: access to laptops (one per group or individual) and crystalmaker software (free download avaialbe)

Implementation Notes: 

This LO was used in a first-year chemistry class at Harvey Mudd College in Fall 2015. I started with a brief lecture (see instructor notes) and then turned the class loose in small groups of about 5 students. I walked through the room to answer questions and guide the groups.

The first day’s activities were taken from a J. Chem. Educ. article (J. Chem. Educ., 2015, 92, 1369-1372). This article has a lot of detail that could be adapted for local use. The related activity "metal and Ionic Lattices Guided Inquiry Worksheet" may be appropriate as review/background material, depending on the placement of this activity in your syllabus.

The second day’s activities rely on the use of crystalmaker, a structure visualization program. There is a free demo version available (http://crystalmaker.com/software/index.html)

Fairly detailed instructor notes are included as a "faculty only" file.

The references for the structures I used are here:

Gibbs G V, Breck D W, Meagher E P (1968)  Structural refinement of hydrous and anhydrous synthetic beryl, Al2(Be3Si6)O18 and emerald, Al1.9Cr0.1(Be3Si6)O18 Note: hydrous emerald. Lithos 1:275-285

Wang X, Hubbard C, Alexander K, Becher P (1994)  Neutron diffraction measurements of the residual stresses in Al2O3 - ZrO2 (CeO2) ceramic composites _cod_database_code 1000059. Journal of the American Ceramic Society 77:1569-1575

I relied on a book called "The science of Color" and a website on color theory (linked below) to develop the 2nd days activities.

The Science of Color,” volume 2, edited by Alex Byrne and David R. Hilbert, MIT Press, Cambridge MA, 1997, pp. 10-17.

Time Required: 
2 50 minute class periods
Evaluation
Evaluation Methods: 

The 2 worksheets were handed in and graded according to the key. I generally used a +, √, - grading scale for the probelms. I gave a single grade for each group. Answer keys are provided as "faculty only" files.

Evaluation Results: 

The day 1 activities were too long and we didn't get to the square planar CFT derivation. For my next offering, I am adding a day to the unit so the students will see all three geometries. Students struggled a bit at first with the software and visualization but were able to figure it out with some assistance. The students in Fall 2015 had already practiced using Crystalmaker in a prior unit; for 2016, this prior unit has been removed so the visualization will probably take more time. I anticipate using 1.5 days for part 1 and 1.5 days for part 2 in Fall 2016.

Creative Commons License: 
Creative Commons Licence

Comments

This LO underwent a fairly significant lengthening (now it is a four day activity) and update for Fall 2016. It covers the same material in much the same way, but now it is much more of a guided inquiry exercise (pre class) with in-class group calculations. I am uploading new versions (LOv2016) for you to look at and evaluate. Reading comes from Atkins, Jones, & Laverman, 7th ed, 2016.


Here's a great general interest article from the New York Times about gemstones: Precious Gems Bear Messages from Earth's Molten Heart by Natalie Angier.

 

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