Molecule Atom Colors – CPK Colors

We’ve all seen models of molecules. Chemistry departments often have kits of plastic balls to stick together to represent atoms and their arrangement in molecules. These kits come with several different colors to easily distinguish one element from another. The choice of color for each atom is not arbitrary. There is a convention matching the color to the elements known as CPK colors.

The CPK part comes from the initials of the scientists who first used the colors to match the elements. Corey, Pauling, and Koltun. In 1952, Robert Corey and Linus Pauling used a basic color code with their models.

Carbon – black
Hydrogen – white
Nitrogen – light blue
Oxygen – red

This color scheme covers the atoms in a great majority of basic organic chemistry molecules, but obviously not all of them. Walter Koltun applied for a patent for a molecule set in 1962 to extend the color list to more elements.

Koltun’s list included

Carbon – black
Hydrogen – white
Nitrogen – blue
Oxygen – red
Sulfur – yellow
Phosphorus – purple
Halogens – green – starting light green for fluorine and getting darker as you move down the group
Metals – silver or gray

There is no set in stone standard for CPK color schemes, but nearly all molecule kits use the same common CPK colors. These colors are typically colors associated with some property of the elements. Sulfur is yellow, carbon is black, chlorine gas is green, iodine gas is purple. Others are less obvious. Nitrogen is found in the sky, and the sky is blue, therefore nitrogen atoms are blue. Oxygen is found in blood, blood is red so oxygen atoms are colored red.

Common CPK Coloring Chart

Today, computers can generate visualizations of molecules with ease. Computers are not limited by the available colors. They can assign distinct colors to every element if they choose. One of the most common software molecule visualization software packages in use today is called Jmol.

Jmol has assigned colors to nearly every element you’re likely to find in a molecule and even a couple of common isotopes. These isotopes include hydrogen’s deuterium and tritium, carbon-13 and carbon-14 and nitrogen-15.

Download the PDF of the JMOL CPK Color Periodic Table showing these colors and their RGB and Hexadecimal values or just read the table below.

CPK Color Sources:

Corey and Pauling’s original model description
Molecular Models of Amino Acids, Peptides, and Proteins
Robert B. Corey and Linus Pauling
Review of Scientific Instruments 24:8, 621-627

Walter Koltun’s US Patent
Space-filling atomic units and connectors for molecular models

Jmol Colors
http://jmol.sourceforge.net/jscolors/