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.

This is a short critical thinking exercise that I use to assess whether my students have understood where the d orbital splitting in Octahedral and Tetrahedral geometry  comes from.  I do it at the beginning of the class after we discuss CFT/LFT in Oh and Td compounds. 

In this activity, a pair of students are show an object or molecule and are asked to determine the point group before their competitor.

Students work in groups to identify relevant steps and intermediates in 3 catalytic cycles, all the while considering bonding (and electron counting) factors.  Following assignment of these steps and intermediate species, the students consider several questions related to catalysis more broadly, particularly the role of each reagent, how to speed up or slow down specific steps, and the importance of regiospecificity in certain steps.