Linus Pauling: Now, why is it that ice has lower density than water; that ice floats? We may say, why is it that ice forms in the world
as it is now, with the Earth the proper, the distance as it is from the sun? This is another peculiarity of water. If we
consider wat-, compare water, hydrogen sulfide, hydrogen selenide, hydrogen telluride, the hydrogen compounds of the elements
in this, group six of the periodic table, we see that hydrogen selenide, hydrogen telluride, hydrogen selenide, hydrogen sulfide,
are gases at room temperature and they, the boiling points, are decreasing in such a way that by extrapolation, we would predict
for water a boiling point of about minus one hundred degrees centigrade rather than plus one hundred degrees centigrade.
It is the fact that hydrogen bonds are formed between the water molecules that causes water to have such a high boiling point
and melting point relative to other substances with the same molecular weight.
The hydrogen bond in water is an interaction. An interaction of the proton of a water molecule, the hydrogen atom of a water
molecule, with an unshared electron pair of another water molecule. And, each water molecule can form four such hydrogen
bonds. Here, a hydrogen bond can be formed using the proton of another oxygen atom, another water molecule, here too, and
here a hydrogen bond using the proton of this oxygen atom.
In order that a hydrogen bond be formed, there must be available an electron pair, an unshared electron pair of one atom,
and a hydrogen attached to another atom that is sufficiently electronegative so that there is some positive charge on the
hydrogen atom. The structure of the hydrogen bond can be thought of as involving in part electrostatic attraction of the
proton when we have, consider, the bond to be ionic, it is about thirty percent ionic, and the electron pair, in part, the
formation of a weak covalent bond between, involving this pair of electrons and the proton when this pair of electrons is
not involved in bonding.
The structure of ice, as determined by x-ray diffraction, is shown by this model. Here, the oxygen atoms are arranged in
a way resembling the arrangement, not identical with, but resembling the arrangement of carbon atoms in diamond. This oxygen
atom is surrounded by four other oxygen atoms at tetrahedron corners. It forms four hydrogen bonds, two using its own hydrogens,
two using the hydrogen atoms of adjacent water molecules.
Closest packing of water molecules would involve ligancy of twelve for each oxygen. Here we have only ligancy four and it
is because of the low value of the ligancy four that this structure, the structure of ice, corresponds to a low density.
The energy of the hydrogen bond, O-H-O, in ice and water, there are still hydrogen bonds in liquid water, about three quarters
as many as in ice. The energy is about five kilocalories per mol of hydrogen bonds, that is about ten per mol of water molecules.
It is this extra stabilization of liquid water and crystalline water that gives rise to the high melting point and boiling
point of the substance, and of course, the hydrogen bond is responsible also for many other properties, characteristic properties,
such as the very high dialectric constant of water. Only the most electronegative atoms form hydrogen bonds: fluorine, oxygen,
The human body is made up largely of compounds of fluorine, not fluorine, of nitrogen and oxygen, and carbon and hydrogen,
and many of these molecules interact with one another through the formation of hydrogen bonds. I believe that as our understanding
of the structure of molecules, of chemical valence, including such weak interactions as the hydrogen bond, becomes more and
more extensive and more and precise, as we obtain a knowledge about the molecular structure of the human body, we shall be
able to make more and more progress in the fields of biology and medicine and that this aspect of chemistry, structural chemistry,
will be found to be, provide the basis for a significant contribution to the welfare of man, to human happiness.