Hydrogen Bonding in Water

Introduction

Strong intermolecular forces called hydrogen bonds are formed between water molecules. Hydrogen bonding is responsible for many of the unusual characteristics of water, namely its relatively high boiling point (and low vapor pressure) for a molecule of its size, the wide range of temperature that this small molecule exists in liquid form, its lower density in the solid form compared with its liquid form, and its propensity to form dome-like droplets on surfaces.

Hydrogen bonds are formed between electronegative atoms of one molecule and hydrogens that are bound to electronegative atoms of another molecule. They have roughly 5% the strength of a covalent bond. For water, the hydrogen bonds form between the oxygen of one water molecule and a hydrogen atom of another water molecule. The typical distance for a hydrogen bond is approximately 2 angstroms.

Press "View" to manipulate a water dimer. Note the angle of the hydrogen bond relative to the oxygen. Try to imagine a hydrogen bond between these two molecules. Where would you place the line representing the bond?

Ice

Ice forms a regular hexagonal packing structure. In ice, the structure is rigid and molecules are held in place by a vast network of hydrogen bonds. Because of the open nature of the hexagonal lattice, ice is less dense than water. Can you complete the hydrogen-bonding network by imagining hydrogen bonds between the water atoms? How many hydrogen bonds are there for each water molecule in this structure of ice?

Liquid water

In water's liquid form, water molecules form extensive hydrogen bonds, on average 2-3 per molecule. Notice that there is not a regular structure to water. It is also important to understand that each hydrogen bond is transient. The large heat of vaporization of water is largely due to the need to break all of the hydrogen bonds in order to produce water vapor.

A maximum of four hydrogen bonds can be formed per water molecule—two for the oxygen and one for each hydrogen. Try to find a water molecule in this structure that is involved in 3 hydrogen bonds.

Theoretical water clusters

It has been postulated that water in its liquid form does form some regular longer-lived structures termed clusters. The graphic displayed by pressing the button below is a theorized icosahedral water cluster. Note the lines in this structure do not represent bonds, but rather geometrical connections between the oxygen atoms in the clustered water molecules. Hydrogen atoms are not shown.

Water and oil don't mix

Water's strong intermolecular hydrogen bonds cause the molecules to associate strongly with each other, and exclude hydrophobic compounds such as oil and lipids to such a degree that the hydrophobic compounds are pushed together. This is what causes oil droplets and oil slicks to form in water. It is also responsible for the formation of lipid micelles (shown in the "View" below), as well as the lipid membranes that make the exterior of cells and cellular organelles.

To understand this structure, you can also take a "slab" half-way through the molecule so that you are looking only at the bottom section of the structure, and can see the nature of the interior of the micelle. Do you see any water molecules on the inside of the micelle?

Water vapor

In water vapor, the water molecules are very separated from each other. Thus, the water molecules rarely interact at a close enough distance to form hydrogen bonds. However, water vapor's behavior is far from that of an ideal gas, in large part because of the strength of intermolecular interactions when two molecules do pass near each other.