, the CPK coloring is a popular color convention for distinguishing atoms
of different chemical element
s in molecular model
s. The scheme is named after the CPK molecular model
s designed by chemists Robert Corey
and Linus Pauling
, and improved by Walter Koltun
In 1952, Corey and Pauling published a description of space-filling models
s and other biomolecule
s that they had been building at Caltech
[Robert B. Corey and Linus Pauling (1953): Molecular Models of Amino Acids, Peptides, and Proteins. Review of Scientific Instruments, Volume 24, Issue 8, pp. 621-627. ]
Their models represented atoms by faceted hardwood balls, painted in different bright colors to indicate the respective chemical elements. Their color schema included
* Sky blue
They also built smaller models using plastic balls with the same color schema.
In 1965 Koltun patented an improved version of the Corey and Pauling modeling technique.
Walter L. Koltun (1965), ''Space filling atomic units and connectors for molecular models''.]
U. S. Patent 3170246
In his patent he mentions the following colors:
* White for hydrogen
* Black for carbon
* Blue for nitrogen
* Red for oxygen
* Deep yellow for sulfur
* Purple for phosphorus
* Light, medium, medium dark, and dark green for the halogen
* Silver for metals (Co
Typical CPK color assignments include:
Several of the CPK colors refer mnemonic
ally to colors of the pure elements or notable compound. For example, hydrogen is a colorless gas, carbon as charcoal
is black, sulfur powder
is yellow, chlorine is a greenish gas, bromine is a dark red liquid, iodine in ether
is violet, amorphous phosphorus
is red, rust
is dark orange-red, etc. For some colors, such as those of oxygen and nitrogen, the inspiration is less clear. Perhaps red for oxygen is inspired by the fact that oxygen is normally required for combustion or that the oxygen-bearing chemical in blood, hemoglobin
, is bright red, and the blue for nitrogen by the fact that nitrogen is the main component of Earth's atmosphere, which appears to human eyes as being colored sky blue.
It is likely that the CPK colours were inspired by models in the nineteenth century. In 1865, August Wilhelm Hofmann, in a talk at the Royal Institution in London, used models made from croquet balls to illustrate valence, so used the coloured balls available to him. (At the time, croquet was the most popular sport in England, so the balls were plentiful.) 'On the Combining Power of Atoms', Chemical News, 12 (1865, 176-9, 189, states that 'Hofmann, at a lecture given at the Royal Institution in April 1865 made use of croquet balls of different colours to represent various kinds of atoms (e.g. carbon black, hydrogen white, chlorine green, 'fiery' oxygen red, nitrogen blue).'
The following table shows colors assigned to each element by some popular software products. Column C is the original assignment by Corey and Pauling,
and K is that of Koltun's patent.
Column J is the color scheme used by the molecular visualizer Jmol
Jmol color table
at sourceforge.net. Accessed on 2010-01-28.
Column R is the scheme used by Rasmol
; when two colors are shown, the second one is valid for versions 2.7.3 and later.
Rasmol color table
at bio.cmu.edu. Accessed on 2010-01-28.
All colors are approximate and may depend on the display hardware and viewing conditions.
* Ball-and-stick model
* Molecular graphics
* Software for molecular modeling
External linksPhysical Molecular Models