In chemistry, the CPK coloring (for Corey–Pauling–Koltun) is a popular color convention for distinguishing atoms of different chemical elements in molecular models.
History
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 White for hydrogen Black for carbon Sky blue for nitrogen Red for oxygen 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."CPK" stands for Corey-Pauling-Koltun. 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 halogens (F, Cl, Br, I) Silver for metals (Co, Fe, Ni, Cu)
Typical assignments
Typical CPK color assignments include: hydrogen (H)whitecarbon (C)blacknitrogen (N)blueoxygen (O)redfluorine (F), chlorine (Cl)greenbromine (Br)dark rediodine (I)dark violetnoble gases (He, Ne, Ar, Kr, Xe, Rn)cyanphosphorus (P)orangesulfur (S)yellowboron (B), most transition metalsbeigealkali metals (Li, Na, K, Rb, Cs, Fr)violetalkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra)dark greentitanium (Ti)greyiron (Fe)dark orangeother elementspink
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.
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.) The essay "On the Combining Power of Atoms", in the 12th volume of Chemical News, 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
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.
Colors 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 Ball-and-stick model Molecular graphics Software for molecular modeling
References
External links Physical Molecular Models