CPK coloring

A plastic ball-and-stick model of proline. These models usually comply with CPK coloring.

In chemistry, the CPK coloring (for CoreyPauling–Koltun) is a popular color convention for distinguishing atoms of different chemical elements in molecular models.


Hofmann's methane model

August Wilhelm von Hofmann was apparently the first to introduce molecular models into organic chemistry, following August Kekule's introduction of the theory of chemical structure in 1858, and Alexander Crum Brown's introduction of printed structural formulas in 1861. At a Friday Evening Discourse at London's Royal Institution on April 7, 1865, he displayed molecular models of simple organic substances such as methane, ethane, and methyl chloride, which he had had constructed from differently colored table croquet balls connected together with thin brass tubes. Hofmann's original colour scheme (carbon = black, hydrogen = white, nitrogen = blue, oxygen = red, chlorine = green, and sulphur = yellow) has evolved into the later color schemes.

In 1952, Corey and Pauling published a description of space-filling models of proteins and other biomolecules that they had been building at Caltech. Their models represented atoms by faceted hardwood balls, painted in different bright colors to indicate the respective chemical elements. Their color schema included

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. In his patent he mentions the following colors:

Typical assignments

brightly colored plastic balls with holes in them.
A box of ball-and-stick model pieces colored to represent several of the common elements.

Typical CPK color assignments include:

hydrogen (H) white
carbon (C) black
nitrogen (N) blue
oxygen (O) red
fluorine (F), chlorine (Cl) green
bromine (Br) dark red
iodine (I) dark violet
noble gases (He, Ne, Ar, Kr, Xe) cyan
phosphorus (P) orange
sulfur (S) yellow
boron (B), most transition metals beige
alkali metals (Li, Na, K, Rb, Cs, Fr) violet
alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra) dark green
titanium (Ti) grey
iron (Fe) dark orange
other elements pink

Several of the CPK colors refer mnemonically to colors of the pure elements or notable compound. For example, hydrogen is a colorless gas, carbon as charcoal, graphite or coke 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.[citation needed]

It is likely that the CPK colours were inspired by models in the nineteenth century. In 1865, August Wilhelm von Hofmann, in a talk at the Royal Institution in London, was using models made from croquet balls to illustrate valence, so he 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)."

Modern variants

Example of Jmol coloring

The following table shows colors assigned to each element by some popular software products.

  • Column C is the original assignment by Corey and Pauling.
  • Column K is that of Koltun's patent.
  • Column J is the color scheme used by the molecular visualizer Jmol.
  • 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.
  • Column P consists of the colors in the PubChem database managed by the United States National Institute of Health.

All colors are approximate and may depend on the display hardware and viewing conditions.

Z Symbol Element C K J R P
1 H hydrogen
1 2H (D) deuterium
1 3H (T) tritium
2 He helium
3 Li lithium
4 Be beryllium
5 B boron
6 C carbon
6 13C carbon-13
6 14C carbon-14
7 N nitrogen
7 15N nitrogen-15
8 O oxygen
9 F fluorine
10 Ne neon
11 Na sodium
12 Mg magnesium
13 Al aluminium
14 Si silicon
15 P phosphorus
16 S sulfur
17 Cl chlorine
18 Ar argon
19 K potassium
20 Ca calcium
21 Sc scandium
22 Ti titanium
23 V vanadium
24 Cr chromium
25 Mn manganese
26 Fe iron
27 Co cobalt
28 Ni nickel
29 Cu copper
30 Zn zinc
31 Ga gallium
32 Ge germanium
33 As arsenic
34 Se selenium
35 Br bromine
36 Kr krypton
37 Rb rubidium
38 Sr strontium
39 Y yttrium
40 Zr zirconium
41 Nb niobium
42 Mo molybdenum
43 Tc technetium
44 Ru ruthenium
45 Rh rhodium
46 Pd palladium
47 Ag silver
48 Cd cadmium
49 In indium
50 Sn tin
51 Sb antimony
52 Te tellurium
53 I iodine
54 Xe xenon
55 Cs caesium
56 Ba barium
57 La lanthanum
58 Ce cerium
59 Pr praseodymium
60 Nd neodymium
61 Pm promethium
62 Sm samarium
63 Eu europium
64 Gd gadolinium
65 Tb terbium
66 Dy dysprosium
67 Ho holmium
68 Er erbium
69 Tm thulium
70 Yb ytterbium
71 Lu lutetium
72 Hf hafnium
73 Ta tantalum
74 W tungsten
75 Re rhenium
76 Os osmium
77 Ir iridium
78 Pt platinum
79 Au gold
80 Hg mercury
81 Tl thallium
82 Pb lead
83 Bi bismuth
84 Po polonium
85 At astatine
86 Rn radon
87 Fr francium
88 Ra radium
89 Ac actinium
90 Th thorium
91 Pa protactinium
92 U uranium
93 Np neptunium
94 Pu plutonium
95 Am americium
96 Cm curium
97 Bk berkelium
98 Cf californium
99 Es einsteinium
100 Fm fermium
101 Md mendelevium
102 No nobelium
103 Lr lawrencium
104 Rf rutherfordium
105 Db dubnium
106 Sg seaborgium
107 Bh bohrium
108 Hs hassium
109 Mt meitnerium
110 Ds darmstadtium
111 Rg roentgenium
112 Cn copernicium
113 Nh nihonium
114 Fl flerovium
115 Mc moscovium
116 Lv livermorium
117 Ts tennessine
118 Og oganesson

See also

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