Why Do Ice Crystals Arrange in a Hexagonal Structure?
Freezing water can form crystals. These crystals regularly assume a hexagonal form. Snowflakes, for example, display a seemingly infinite variety of patterns, but all these patterns are variations of the basic hexagon theme. The reason for this lies in the structure of the water molecule.
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Water Molecule
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The water molecule roughly approximates a tetrahedron in form, with a large oxygen atom in the center and two small hydrogen atoms perched at two corners of the tetrahedron. Two sets of paired electrons occupy the other two tetrahedral corners. The angles between each of these four corners are fairly close to the tetrahedral angle of 109.5 degrees.
Hydrogen Bonds
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The electrical charge of the water molecule is unevenly distributed. The side with the two hydrogen atoms is positive and the side with the paired electrons is negative, so each hydrogen atom will weakly attract the negative side of a neighboring water molecule. These weak attractions are called hydrogen bonds.
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Hexagonal Crystals
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Because of the angle between the hydrogen atoms of each water molecule, the mutually bonded molecules form a hexagonal lattice when frozen. This basic hexagonal lattice on a molecular level explains why ice crystals grow into a hexagonal structure.
Why Hexagons?
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Dr. Angelos Michaelides and a team of scientists discovered pentagonal ice crystals on copper. Heptagons also occur. Why are hexagons the common arrangement? "There is no a priori rule that hexagons should form," according to Dr. Michaelides. They just do.
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References
- University of Wisconsin Stevens Point: Ice Structure
- The Naked Scientists: The Science of Snowflakes
- Internet Archive: Hydrogen Disorder in Ice
- Chem 1: A Gentle Introduction to Water and Its Structure
- Internetchemistry.com: It's Raining Pentagons
- California Institute of Technology: A Snowflake Primer
Resources
- Photo Credit snowflake image by Egor Tkachenko from Fotolia.com
