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, and nitrogen.
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.
