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CPK coloring

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

In chemistry, the CPK coloring is a popular color convention for distinguishing atoms of different chemical elements in molecular models. The scheme is named after the CPK molecular models designed by chemists Robert Corey and Linus Pauling, and improved by Walter Koltun.

History[edit source | edit]

In 1952, Corey and Pauling published a description of space-filling models of proteins and other biomolecules that they had been building at Caltech.[1] 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.[2] In his patent he mentions the following colors:

Typical assignments[edit source | edit]

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, Xe, Kr) cyan
     phosphorus (P) orange
     sulfur (S) yellow
     boron (B), most transition metals peach, salmon
     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.

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).'[3] [4] [5]

Modern variants[edit source | edit]

Example of CPK 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,[1] and K is that of Koltun's patent.[2] Column J is the color scheme used by the molecular visualizer Jmol.[6] 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.[6][7] All colors are approximate and may depend on the display hardware and viewing conditions.

A# Sy Element C K J R
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[edit source | edit]

References[edit source | edit]

  1. 1.0 1.1 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. doi:10.1063/1.1770803
  2. 2.0 2.1 Walter L. Koltun (1965), Space filling atomic units and connectors for molecular models. U. S. Patent 3170246.
  3. https://books.google.co.uk/books?id=-PjNAAAAMAAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q=fiery&f=false
  4. Historical Studies in the Language of Chemistry', M.P.Crossland, 1962, page 336, and footnote 220 on page 336.
  5. https://books.google.co.uk/books?id=kwQQaltqByAC&pg=PA336&lpg=PA336&dq=%27On+combining+power+of+atoms%27+chemical+news+1865&source=bl&ots=Z9e14A0ykR&sig=ACfU3U0njHT4Cpw24pHCYyR98zXiGUiDjA&hl=en&sa=X&ved=2ahUKEwjSio-EruDnAhVPiFwKHZW3CgMQ6AEwAHoECAcQAQ#v=onepage&q='On%20combining%20power%20of%20atoms'%20chemical%20news%201865&f=false
  6. 6.0 6.1 Jmol color table at sourceforge.net. Accessed on 2010-01-28.
  7. Rasmol color table Archived 2001-05-13 at Archive.today at bio.cmu.edu. Accessed on 2010-01-28.

External links[edit source | edit]