A Cache of Chemistry Models
While today’s undergrad can conjure up a molecular structure on a computer screen in a matter of seconds, for most of the history of scientific education physical atomic models have been basic building blocks for teaching and understanding chemistry. And Caltech has played a key role in that history.
Early models, dating back as far as the 1860s, featured balls representing atoms and sticks signifying chemical bonds. In the 1950s, Caltech’s Robert Corey and Linus Pauling, along with UC Berkeley’s Walter Koltun, led the design of a new type of three-dimensional model. These space-filling calotte models (also known as CPK models), which became standard issue not only in laboratories but in science classrooms the world over, are made up of individual balls that represent atoms; the size of each sphere is proportional to the size of the actual atom, and its color linked to the type of atom.
Not only were the molded-plastic CPK models developed at Caltech commercially successful, they also established a popular color convention for atoms in molecular models, called the CPK Coloring Scheme—with red for oxygen, blue for nitrogen, white for hydrogen, yellow for sulfur, and black or gray for carbon.
A sizable collection of CPK model components can still be found on the Caltech, largely due to the efforts of Larry Henling, staff crystallographer in Caltech’s X-Ray Crystallography Facility. Henling has preserved drawer upon drawer of the molded-plastic atoms and their connector links as well as design drawings, blueprints, contemporary photos, and correspondence related to the models.
Does he have a favorite? “Glycine,” says Henling. “It is an amino acid, the simplest one, but wasn't easy to solve at the time the structure was done. In the crystal it is a zwitterion—an overall neutral molecule but with a positive charge at one end and a negative at the other."
Shelves and display cases in Henling’s office show off other models that have found their way to Caltech over the years—the ball-and-stick and skeleton variety, as well as more unusual cardboard iterations—while a table near the window of the crowded space is given over to an outsize model of copper metal used by Linus Pauling in lectures.
Though Henling can’t recall when the models were last used, it feels important to him to keep them. “The models and blueprints are historical reminders of the time and effort Caltech researchers, many of whom are now forgotten, put into developing an understanding of molecular structures,” Henling says. “Today, scientists do the same with just a push of a computer button.”