Physics:CPK coloring

Short description: Colour convention for differentiating atoms

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

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

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

• 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. 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

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.

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.

			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

External links

• Physical Molecular Models