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_ N_Water and Ice
    
 Table of Contents
  Introductory Comments
  What is Molecular Modeling?
  Why is Molecular Modeling Important?
  What do some common molecules look like?
  Where's the Math?
  Carbon 3 Ways
  Carbon Compounds
  Water and Ice
  Water and Ice pt.II
  How to view structures in class or at home
  MathMol Library of Structures.
  Tutorial 1: 1-Dimension, 2-Dimensions, 3-Dimensions...
  Tutorial2: The Geometry of 2 Dimensions..
  Tutorial3: The Geometry of 3- Dimensions
  Tutorial4: The Geometry of Molecules.
  Appendix1: Scientific Notation
  Appendix 2: Mass
  Appendix3: Volume
  Appendix4: Density
 

Water has important effects on all biological systems. What makes water so unique are two very important properties.

Water is a polar molecule

A water molecule is formed when two atoms of hydrogen bond covalently with an atom of oxygen. In a covalent bond electrons are shared between atoms. In water the sharing is not equal. The oxygen atom attracts the electrons more strongly than the hydrogen.This gives water an asymmetrical distribution of charge. Molecules that have ends with partial negative and positive charges are known as polar molecules. It is this polar property that allows water to separate polar solute molecules and explains why water can dissolve so many substances.


Water is highly cohesive .

The positive regions in one water will attract the negatively charged regions in other waters. The dashes show the hydrogen bond. In a hydrogen bond a hydrogen atom is shared by two other atoms. The donor is the atom to which the hydrogen is more tightly linked. The acceptor (having a partial negative charge) is the atom which attracts the hydrogen atom. Click here or on the image to your left to view a movie of two water molecules.

Liquid water has a partially ordered structure in which hydrogen bonds are constantly being formed and breaking up.

See a flash movie of water molecules in action.

Hydrogen bonds are much weaker than covalent bonds. However, when a large number of hydrogen bonds act in unison they will make a strong contributory effect. This is the case in water.

View Flash animation of this image.


On the other hand ice has a rigid lattice structure.

In liquid water each molecule is hydrogen bonded to approximately 3.4 other water molecules. In ice each each molecule is hydrogen bonded to 4 other molecules.

Compare the two structures below. Notice the empty spaces within the ice structure.
Image   Image
To View Water and Ice Side-by-Side in 3-D: Click here.

In ice Ih, each water forms four hydrogen bonds with O---O distances of 2.76 Angstroms to the nearest oxygen neighbor. The O-O-O angles are 109 degrees, typical of a tetrahedrally coordinated lattice structure. The density of ice Ih is 0.931 gm/cubic cm. This compares with a density of 1.00 gm/cubic cm. for water.

There are eleven different forms of crystalline ice that are know. The hexaganol form known as ice Ih is the only one that is found naturally. The lattice structure of ice 1h is shown here.

Image


Go to Water PartII__

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