14 Jul 2014

The Structure and Color of Alums

In-Class Activity

Submitted by Erica Gunn, Simmons College

This is an in-class assignment designed to help students integrate their understanding of periodic trends and materials properties. Using the color of alum crystals as an example of octahedral coordination chemistry, students use their knowledge of electronic structure and periodic trends to predict which of the isomorphous alum crystals will be colored, and to qualtitatively rank the degree of crystal field splitting in a family of alum crystals.

Learning Goals: 

In answering these questions, a student will:

- Apply knowledge of electron configurations to predict which metal ions will produce color.

- Use the spectrochemical series to determine whether the metal ions in an alum crystal will be high or low spin

- Compare field splitting for different alum compositions based on periodic trends


Implementation Notes: 

This worksheet was developed for a second-year descriptive inorganic chemistry class. Students have been introduced to periodic trends, electron configurations for transition metals, crystal structure and site symmetry, and are beginning a discussion of crystal field theory. They should also have been exposed to the spectrochemical series and ligand field splitting. This worksheet is used as a way of integrating knowledge across several chapters and several weeks of the semester, in preparation for a more advanced discussion of color in pigments and gemstones. After a brief introduction, students will work in small groups during class time to answer the worksheet questions. The questions follow closely with content in the textbook (Descriptive Inorganic Chemistry Raynor-Canham, Overton 6th ed.), and so should be mostly a review and opportunity for students to apply their understanding to a concrete problem.

Time Required: 
1 hour
Evaluation Methods: 

This LO has not yet been tested. 

Students could be asked to summarize:

a. the relevance of periodic trends to the color of alum crystals

b. why potassium alum is colorless and chrome alum is colored

c. how the crystal field splitting varies with periodic trends, and how this is (or is not) related to the color of the alums

d. rank the predicted field splitting values for a different series of metal impurities or different set of isomorphous crystals

e. explain why crystal field theory is limited in its ability to predict color in the alum crystals (only after a more advanced discussion of ligand field theory - see instructor notes)


This data could be collected in several different ways:

a. Class discussion or review in a future lecture period

b. Quiz, exam, or final exam question

c. Minute paper or clicker question



Evaluation Results: 

This LO  has not yet been assessed.

Creative Commons License: 
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