Hydogen bonding is of an electrostatic origin and can be considered a special type of dipole-dipole interaction. When hydrogen is covalently bonded to an electronegative atom such as oxygen or nitrogen it has a net partial positive charge. The positive partial charge on hydrogen can attract another electronegative atom such as oxygen or nitrogen. A simple example is the water dimer shown below.

		What is the length of a hydrogen bond? How does the length of a hydrogen bond compare to that of a covalent bond? Measure the distance of the covalent bond between oxygen and hydrogen. (You may have to zoom in first --left mouse button and shift key--). 1) click the right mouse button ---> Select --> Mouse Click Action ---> Toggle Distance Monitor ---> Click 2) Click on the two atoms e.g., oxygen covalently bound to hydrogen, or oxygen and hydrogen in a hydrogen bond. Check your values: hydrogen bond length. covalent hydrogen-oxygen bond.
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What is the distance of a covalent bond formed between oxygen and hydrogen in a water molecule?: Angstroms What is the distance of a hydrogen bond formed between oxygen and hydrogen in a water dimer? Angstroms

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. In water hydrogen bonds are constantly being formed and broken.Click here for a movie that demonstrates this. The time scale is about 10 picoseconds.

		When the temperature of liquid water is lowered the speed of molecules slows increasing the time that hydrogen bonds last. The results in the rigid lattice structure known as ice.

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.

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.

		The hydrogen bonds form a tetrahedral array in ice crystals.
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