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. 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

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!




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.