A Sweet Briar College Learning Resource

H2O - The Mystery, Art, and Science of Water

The Chemistry of Water
Professor Jill Granger

Structure Means Function

Water is a Chemical!?

Indeed! Water is one of our most plentiful chemicals. Its chemical formula, H20, is probably the most well known of all chemical formulas.


What does the chemical formula tell us?

The formula H20 tells us that one molecule of water is comprised of 2 atoms of hydrogen and one atom of oxygen bonded together. The bonds which hold the hydrogen and oxygen together are called covalent bonds - they are very strong.

Let's look at a picture of a molecule of water: In this picture the two hydrogens are represented by white spheres and the oxygen by a red sphere.


IMAGE SOURCE: "Chemistry and Life", 4th Edition, John W. Hill, Dorothy M. Feigl, and Stuart J. Baum, Macmillan Publishing Company, New York, 1993

In this second picture, the hydrogens are shown as white spheres, the oygen as a red sphere. The 'sticks' holding the hydrogens to the oxygen represent covalent bonds.


Why does the water molecule look bent?

The water molecule maintains a bent shape (bent at 107.5 degrees actually) because of two considerations. First the tetrahedral arrangment around the oxygen and Second the presence of lone pair electrons on the oxygen.


What are Lone Pair Electrons?

These are the electrons that are not involved in the covalent bonds. The pairs of electrons are left alone. In our picture they are represented by the double dots. These lone pairs are very negative - containing two negative electrons each - and want to stay away from each other as much as possible. These repulsive forces act to push the hydrogens closer together


Did you say "Tetrahedral" - What does that mean?

Tetrahedral means "four-sided". In chemistry we interpret this in our imaginations. Draw the central atom in an imaginary space. Next put the atoms attached to the central atom around it such that the distance between them is maximized. The arrangement you'll adopt will be the form of a regular tetrahedron. This molecular shape is shown below. It has regular bond angles of 109.5


IMAGE SOURCE: "Chemistry and Life", 4th Edition, John W. Hill, Dorothy M. Feigl, and Stuart J. Baum, Macmillan Publishing Company, New York, 1993

If we do a similar arrangement of water, putting oxygen in the center, and using the two hydrogens and two lone pairs at the corners, we also come up with a tetrahedral arrangement. However, there is one important difference - the bond angles for water are not 109.5. Because of the presence of the very negative lone pair electrons, the two hydrogens are squeezed together as the two lone pairs try to get away from each other as far as possible. The resulting angle gives water a 104.5 bond angle. Because we don't "see" the electrons, the resulting tetrahedron "looks" BENT!


IMAGE SOURCE: "Chemistry and Life", 4th Edition, John W. Hill, Dorothy M. Feigl, and Stuart J. Baum, Macmillan Publishing Company, New York, 1993


What's your Point?

Like many things in the chemical world, the shape and structure of a molecule is an important determinant of its function. The importance of the bent structure of water is that it provides water with two distinct "sides": One side of the water molecule has two negative lone pairs, while the other side presents the two hydrogens. Let's take another look:

[ fig of electron density map of water ]


Does this make water unusual?

YES! But it's not just that the molecule is bent that makes it special. Water is also highly polar - the two sides of water have very different charge.


The lone pairs are negative - Are the Hydrogens positive?

The hydrogens are slightly positive. They get this way because of the "electronegativity" of oxygen. Electronegativity is a measure of how much one atom wants to have electrons, and oxygen wants to have electrons more than hydrogen does. Oxygen has a higher electronegativity. Because of this difference in electronegativity, the electrons in the covalent bonds between oxygen and hydrogen get pulled slightly toward the oxygen. This leaves the hydrogens a little bit electron-deficient and thus slightly positive. We can draw this polarization like this:


IMAGE SOURCE: "Chemistry in Context" Wm C Brown Publishers, Dubuque Iowa, 2nd edition, A project of the American Chemical Society, ed: A. Truman Schwartz et al., 1997, Chapter 5 "The Wonder of Water"

Or looking at it from a "net polarization" perspective, like this:


IMAGE SOURCE: "Chemistry and Life", 4th Edition, John W. Hill, Dorothy M. Feigl, and Stuart J. Baum, Macmillan Publishing Company, New York, 1993


What does the polarization have to do with the properties of water?

Everything! Because water has a slightly negative end and a slightly positive end, it can interact with itself and form a highly organized 'inter-molecular' network. The positive hydrogen end of one molecule can interact favorably with the negative lone pair of another water molecule. This interaction is call "Hydrogen Bonding". It is a type of weak electrostatic attraction (positive to negative). Because each and every one of the water molecules can form four Hydrogen Bonds, an elaborate network of molecules is formed.


IMAGE SOURCE: "Chemistry in Context" Wm C Brown Publishers, Dubuque Iowa, 2nd edition, A project of the American Chemical Society, ed: A. Truman Schwartz et al., 1997, Chapter 5 "The Wonder of Water"


But if the Hydrogen Bonds are weak, how can they be important?

Think of how many there are! There is strength in numbers!

The polarity also allows water interact with an electric field:

And to interact with other polar molecules - which is how substances become dissolved in water.


IMAGE SOURCE: "Chemistry in Context" Wm C Brown Publishers, Dubuque Iowa, 2nd edition, A project of the American Chemical Society, ed: A. Truman Schwartz et al., 1997, Chapter 5 "The Wonder of Water"




Selected by the SciLinks program, a service of National Science Teachers Association. Copyright 1999 - 2002





CONTENTS INTRODUCTION PURPOSE SCHEDULE REQUIREMENTS PARTICIPANTS


H20 - The Mystery, Art, and Science of Water
Chris Witcombe and Sang Hwang
Sweet Briar College