Plato assumed these shapes corresponded to the properties given; in particular associating icosahedra with water (as I do in this web site).a They are the only regular solids where all the vertices and the centers of all the faces and edges lie on spheres (the circumscribed, inscribed and mid spheres respectively) with the same center.
The properties of these solids, with edge length (el) are given in the following table:
Name |
Coordinates, el [444] |
|---|---|
(-½√½,½√½,½√½)(½√½,-½√½,½√½) |
|
(±½, ±½, ±½) |
|
(±√½, 0, 0)(0, ±√½, 0)(0, 0, ±√½) |
|
(0, ±½,
±½(1+φ))(±½(1+φ),
0, ±½) |
|
(±½, 0, ±φ/2)(±φ/2, ±½, 0)(0, ±φ/2, ±½) |
where φ is the golden ratio. A rectangle with sides in the ratio 1:φ gives a similar rectangle when the square side 1 is removed:
φ = (√5+1)/2 = 1.618034....
1/φ = φ - 1= (√5-1)/2
= 0.618034....
φ2 = φ + 1= (√5+3)/2
= 2.618034....
φn+1 = φn + φn-1
The golden ratio occurs in the dimensions of the pentamers of water molecules that are commonly found in liquid water and the water icosahedra described at this site. Thus the ratio of the distances between the nearest-neighbor water molecules (a) and between the next to nearest-neighbor water molecules (b) in planar water hydrogen-bonded pentamers (H2O)5 is
2 x sin(108°/2) = φ = (√5+1)/2 = 1.618034.... . Also these diagonals intersect each other in the golden ratio (a/c = φ).
Interestingly the golden ratio also appears in aqueous chemistry as the ratio between atomic and ionic diameters. Thus the diameter of an anion (A-) is twice its atomic diameter divided by φ and the diameter of a cation (A+) is twice its atomic diameter divided by φ2; with the diameter of A- being the golden ratio times the diameter of A+, and simple functions of φ also relating ion-water distances to covalent radii [1091]. The golden ratio has also been asociated with the genetic code [1808].
Plato would not have been wrong to connect liquid structure in general to icosahedra as spherical atoms and molecules (for example, the larger noble gases) in the liquid phase prefer icosahedral clustering which has a lower energy than crystal structures (but cannot form crystals due to the five-fold symmetry).
Shown right is an icosahedral cluster of thirteen identical spherical atomsb as found in liquid argon, krypton, xenon and molten metals; such five-fold symmetry being optimal for short-range close packing but incompatible with long-range order and favoring amorphous structures. Its preferred formation has been shown to prevent crystallization in liquid metal melts and be the cause of their extensive supercooling [505].
It is clear
from the evidence
presented at this site that water
may form icosahedral clusters,
so linking modern science with ancient philosophy.
Below is a Java appletc showing the solid shape of the proposed water icosahedral cluster (H2O)280. It is a truncated icosahedron with 12 pentagon faces (with edge length el ~ 0.28 nm), 20 equilateral triangular faces (with edge length 4 x (2/3)1/2 x el ) and 30 rectangular faces (with edge lengths el and 4 x (2/3)1/2 x el ). (Note that 4 x (2/3)1/2 is 3.266 and close to the value of 2φ).
a The association of the dodecahedron with the Universe has also received a recent burst of interest, now somewhat subsiding [1163]. [Back]
b One atom resides in the slightly-too-small cavity at the center, causing loose contact between the twelve at the vertices. Note that thirteen atoms can only fit snugly together in a cuboctahedron (with 8 triangular and 6 square faces) formed from three layers containing 3, 7 and 3 atoms and part of a hexagonal close packed arrangement. [Back]
c This uses a non-commercial Java 1.1 applet by Martin Kraus. Use the mouse to rotate the structure. [Back]
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This page was last updated by Martin Chaplin on 15 September, 2012
