7.5. Special properties of water[[Student version, January 17, 2003]] 241
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HH 0.27 nm
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OFigure 7.12:(Sketch; metaphor.) (a)Tetrahedral arrangement of water molecules in an ice crystal. The heavy lines
depict chemical bonds; the dashed lines are hydrogen bonds. The gray outlines of the tetrahedron are just to guide
the eye. The oxygen atom in the center of the figure has two dashed lines (one is hidden behind the oxygen), coming
from the directions maximally distant from the directions of its own two hydrogen atoms. (b)Crystal structure of
ice. [From Ball, 2000.]
There are many small polar molecules, of which many are dipoles. Among these, water belongs
to an even more special subclass. To see why, note that each hydrogen atom in a water molecule had
only one electron to begin with. Once it has lost that electron, each hydrogen ends up essentially
anaked proton; its physical size is much smaller than that of any neutral atom. The electric field
about a point charge grows as 1/r^2 as the distancerto the charge goes to zero, so the two tiny
positive spots on the water molecule are each surrounded by an intense electric field. This effect is
specific to hydrogen: Any other kind of atom bonded to oxygen retains its other electrons. Such a
partially stripped atom carries about the same charge +eas a proton, but its charge distribution
is much larger and hence more diffuse, with milder electric fields than those on a hydrogen.
Each water molecule thus has two sharply positive spots, which are oriented at a definite angle
of 104◦to each other. That angle is roughly the same as the angle between rays drawn from the
center to two of the corners of a tetrahedron (Figure 7.12a). The molecule will try to orient itself
in such a way as to point each of its two positive spots directly at some other molecule’s “back side”
(the negatively charged region opposite the hydrogens), as far away as possible from the latter’s two
positive spots. The strong electric fields near the hydrogen atoms make this interaction stronger
than the generic tendency for any two electric dipoles to attract, and align with, each other.
The idea that a hydrogen atom in one molecule could interact with an oxygen atom in another
one, in a characteristic way, was first proposed in 1920 by M. Huggins, an undergraduate student of
the chemist G. Lewis. Lewis promptly christened this interaction thehydrogen bond,orH-bond.
In a sample of liquid water, every molecule will simultaneously attempt to point its two hydrogen
atoms toward the back sides of other molecules. The best way to arrange this is to place the water
molecules at the points of a tetrahedral lattice, as in Figure 7.12. The left panel shows a central
water molecule with four nearest neighbors. Two of the central molecule’s hydrogens are pointing
directly at the back sides of neighbors (top and front-right), while its two other neighbors (front-
left and back) in turn pointtheirhydrogens towardsitsback side. As we lower the temperature,
thermal disorder becomes less important, and the molecules lock into a perfect lattice—an ice
crystal. To help yourself imagine this lattice, think of your torso as the oxygen atom, your hands as
the hydrogen atoms, and your feet as the docking sites for other hydrogens. Stand with your legs
apart at an angle of 104◦,and your arms at the same angle. Twist 90◦at the waist. Now you’re