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Journal ArticleDOI

The constitution and fundamental properties of solids and liquids. part i. solids.

01 Nov 1917-Journal of The Franklin Institute-engineering and Applied Mathematics (American Chemical Society)-Vol. 184, Iss: 5, pp 721
About: This article is published in Journal of The Franklin Institute-engineering and Applied Mathematics.The article was published on 1917-11-01. It has received 7517 citations till now. The article focuses on the topics: Constitution.

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CONSTITUTION
OF
SOLIDS
AND
LIQUIDS.
222
I
Bismuth.-Oeschsner de Coninck and Gsrard,' by reduction of bismuth
Molybdenum.-Miiller,2 by oxidation of the metal, found
Mo zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
=
96.035.
Neodymium.-Baxter, Whitcomb, Stewart, and Cha~in,~ by analyses
Columbium.-Smith and Van Haager~,~ from the ratio between sodium
The value
Argon.-Schultze5 has redetermined the density of argon. The cor-
chloride to metal, found Bi
=
208.50.
of the chloride, find Nd
=
144.27.
columbate (NaCbOJ and sodium chloride, find Cb
=
93.13.
93.1
might be adopted in the table.
responding atomic weight is zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
A
=
39.945.
Signed,
F.
W. CLARKE,
T.
E.
THORPE,
G. URBAIN.
NOTE.-Because of the European war the Committee has had much
difficulty in the way of correspondence. The German member, Pro-
fessor Ostwald, has not been heard from
in
connection with this report.
Possibly the censorship of letters, either in Germany or zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
en
route,
has led
to a miscarriage.
F.
W. CLARKE,
Chairman.
[CONTRIBUTION FROM
RESEARCH
LABORATORY
OF
THE
GEXERAL
ELECTRIC
COMPANY,
SCHENECTADY,
N.
Y.]
THE CONSTITUTION AND FUNDAMENTAL PROPERTIES
OF
SOLIDS AND LIQUIDS.
PART
I.
SOLIDS.
BY
IRVING
LANGMUIR.
Received
September zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
5, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
1916.
The importance of the work of W.
H.
Bragg and
W.
I,.
Bragg in its
bearing on chemistry has not, as yet, been generally recognized.
In
hearing two of W.
H.
Bragg's lectures in this country a few years ago, the
writer was impressed with the very great significance of this work in the
field of chemistry. The structure of crystals as found by the Braggs
leads to new and more definite conceptions as to the nature of chemical
forces.
The writer has constantly endeavored to apply this new conception
in his work
on
heterogeneous reactions and particularly in connection with
a study of the phenomena of adsorption and surface tension. In this way
he has gradually been led to form more or less definite theories of the
mechanism
of
evaporation, condensation, liquefaction, adsorption, and
capillary phenomena. According to this theory, both solids and liquids
Compt.
rend.,
162,
252
(1916).
THIS
JOURNAL,
37, 2046 (1915).
Ibid.,
38, 302 (1916).
Ibid.,
38, 1783 (1916).
E. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
Ann.
Physik,
[iv]
48, 269 (1915).

2222 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
IRVING zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
LANGMUIR.
consist
of
atoms held together entirely by zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
chemical
forces.
The conception
of
the molecule thus almost entirely loses its significance except in the case
of gases. In fact, we may well look upon any solid or liquid body as con-
stituting a single large molecule. Any change of phase, such as the melt-
ing
of
a solid, is thus a typical chemical reaction. The mobility of liquids,
according to this viewpoint, is due to a kind of tautomerism.
The present paper is merely an outline
of
this theory.
The more de-
tailed description of the experimental work upon which
it
is largely based,
will be reserved for future papers.
STRUCTURE
OF
CRYSTALS.
The idea that
a
crystal should constitute an effective diffraction grating
for X-rays originated with Laue,l who also saw that by means
of
such a
grating not only could the wave length
of
the X-rays be determined, but
also a powerful method for studying the structure of crystals was made
available. The detailed theory developed by Laue proved incorrect in
certain important respects.
W.
L. Brag$ called attention to these errors
and gave a theory by which the true structure could be determined from
the diffraction pattern. Since then the method has been greatly simplified,
so
that at present it is usually a matter of no great difficulty to find the
exact arrangement of the atoms in a crystal. In this way the structures
of
some
30
or more kinds of crystals have been determined.
The alkali halides NaC1, KC1, KBr, and
KI
all have a similar structure
in which the zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
atoms
are arranged according to a simple cubic lattice. Thus,
in the case of sodium chloride crystals, sodium and chlorine atoms alter-
nate along three sets
of
lines at right angles to each other. Each sodium
atom is surrounded by six equidistant chlorine atoms arranged around
it, as the corners
of
an octahedron are arranged around its center. Simi-
larly, each chlorine atom is surrounded by six equidistant sodium atoms.
Up to this time it had been taken for granted that crystals were built
up
of
molecules. But from this work
of
the Braggs
it
is clear that in
crystals of this type the identity of the molecules is wholly lost, except
in
so
far as we may look upon the whole crystal as composing a single
molecule.
From
the arrangement of the atoms we must conclude that the
forces holding the crystal together (cohesion) are forces which exist
directly between sodium and chlorine atoms. Every chemist looks upon
such forces as chemical in nat~re.~
1
Sitzb.
d.
Bayer.
Akad.
d.
Wiss.,
June,
1912.
2
Proc.
Camb. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
Phil. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
Soc.,
17,
43 (1912).
The subsequent papers of
W.
H.
and
W.
I,.
Bragg were published in 1913 and 1914 in the
Proc.
Camb.
Phil. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
SOC.,
the
Proc.
Roy.
Soc.,
and the
Phil.
Mag.
All of these papers
were
translated into German
and were published together in
one
volume of the
Z.
anorg.
Chem.,
go
(1914).
3
Of course all chemical forces are probably
of
electromagnetic origin, but for this
reason
we
do not need to call the forces in crystals by the vague term “physical forces”
(and thus distinguish them from chemical force).

CONSTITUTION
OF
SOLIDS AND LIQUIDS.
2223
But the significance of this structure for the chemist extends further.
Sodium is invariably regarded as a monovalent element, yet in the sodium
chloride crystals we see a structure which in no wise suggests the mono-
valent character of the atom. The sodium atom is held by chemical
forces to six chlorine atoms.
If
we retain the conception of valency in
such a case, we must clearly admit that the valency of the sodium is
divided equally between the six chlorine atoms.
If
a sodium chloride crystal evaporates at high temperature, the atoms
leave the surface in pairs in the form
of
sodium chloride molecules. These
molecules are formed in the process of evaporation, since they do not
exist as such in.the crystal. The process of evaporation is thus a chemical
process.
In the diamond, each carbon atom
is
surrounded by four others equi-
distant from it. These are arranged around the central one in the same
way as the four corners
of
a
regular tetrahedron are arranged around its
center. In this case the tetravalent character of the carbon atom mani-
fests itself clearly. When
a
model of a diamond crystal is examined, it is
seen that the atoms appear to be arranged in rings of six, corresponding
to the benzene ring. The remarkable strength of the carbon chain and
especially the stability of the benzene ring,
so
familiar to chemists, is
thus seen to be the cause of the hardness, the high melting point and the
low vapor pressure of the diamond.
It
is interesting to note that the structure of zinc blend, ZnS, is very
similar to that of the diamond, the zinc and sulfur being alternately sub-
stituted for adjacent carbon atoms. Thus each zinc atom is surrounded
by four symmetrically placed sulfur atoms, while each sulfur atom is
surrounded by four zinc atoms. The valency is thus again divided.
With fluorite, CaF2, each fluorine atom is surrounded by four sym-
metrically placed calcium atoms, while each calcium atom has eight
fluorine atoms arranged around it like the eight corners of
a
cube about
the center of the cube.
Each iron
atom has four equidistant sulfur atoms around it, but there are other
sulfur atoms at distances only slightly greater. Similarly, each sulfur
atom has three equidistant iron atoms forming a triangle around it, but
above and below the plane of this triangle there are other iron atoms
whose distance is only slightly greater.
The structure of pyrites, FeS2, is much more complicated.
Hauerite, MnS2, has
a
similar structure to that of pyrites.
In the case
of
calcite, CaC03, the carbon and oxygen atoms lie in planes
perpendicular to the crystal axis. Three oxygen atoms are arranged in
groups around each carbon atom, forming an equilateral triangle. The
calcium atoms lie in planes just above and just below the carbon-oxygen
planes. Each calcium atom has zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
six
equidistant oxygen atoms around it.

2224 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
IRVING LANGMUIR.
The distance between the calcium and the carbon atoms is considerably
more than either the distance between oxygen and carbon, or between
oxygen and calcium. Thus the crystal
is
evidently held together by forces
acting between carbon and oxygen and between oxygen and calcium.
The group CO3 appears as
a
unit, since each carbon atom is associated
with three oxygen atoms while these are associated with
only
the one
carbon atom. But each CO3 group is equidistant from six calcium atoms,
so
that it is impossible to pick out any one of these as forming a molecule
with the COS.
If
we are to retain the idea of a molecule at all, we must
consider that the entire crystal is a single molecule.
In
dolomite, CaMgC03, the structure is exactly similar except that
calcium and magnesium planes alternate. The structures of rhodo-
chrosite, MnC03, siderite, FeC03, and sodium nitrate, NaN03, are like
that of calcite. This last case is of special interest to the chemist, since a
monovalent atom replaces a divalent, and a pentavalent zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
(?)
atom replaces
a tetravalent one without causing a change in the arrangement of the
atoms.
The crystals of magnetite afford an illustration of a structure in which
different atoms of the same element have different 'functions. Two-
thirds of the iron atoms occur in positions in which each is surrounded
by four oxygen atoms (tetrahedral arrangement), while the other third
of the iron atoms are each surrounded by six oxygen atoms. The iron
atoms have thus a divalent and trivalent character, but each unit of
valence is divided between two oxygen atoms. Spinel, MgA1204, has an
exactly similar structure in which the Mg atoms take the place of the
divalent iron atoms, while the A1 atoms replace the trivalent iron atoms.
Crystals
of
metallic copper and metallic silver have been found to have
their atoms arranged according to the face-centered cubic lattice. This
arrangement is the same as the familiar one obtained when round shot
are piled in layers as regularly and compactly as possible.
In
this struc-
ture each atom is equidistant from the twelve adjacent atoms.
The structures of crystals
of
rhombic sulfur and quartz have been
partly worked out.
In
the sulfur crystals the atoms are found to be
arranged in a lattice structure in groups of eight. The particular arrange-
ment of the atoms within these groups is not yet known. To the chemist
it is of significance that these groups of eight contain the same number
of atoms as are found in molecules of sulfur vapor. Here, then, for the
first time, the crystal has a structure in which the identity of the mole-
cules (as found in the gas phase) is apparently not wholly lost.
In
a recent article'
I,.
Vegard shows that the structures of gold and lead
crystals are the same as those
of
silver and copper. He also describes
the structure of the zircon group of minerals represented by zircon, ZrSiOl,
1 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
Phil. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
Mag., zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
32,
65
(1916).

CONSTITUTION
OF zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
SOLIDS
AND LIQUIDS.
2225
rutile, Ti02, and cassiterite, SnOz. The structure of zircon proves to be
especially interesting, for it is found “that each of the
Zr
or Si atoms is
associated with two oxygen atoms; thus the groups Si02 and ZrOz form a
kind of ‘molecular elements’
of
the lattice. This is not merely a way of
regarding the geometrical arrangement of the atoms; but we have reason
to believe that the groups Si02 and ZrOz form chemically saturated com-
pounds’’ within the crystal.
Two facts stand out clearly as a result of the consideration of these
crystal structures. In the first place, it is evident that crystals are
built up of atoms in such a way that each atom is chemically combined to
all the adjacent atoms, while these in turn are combined to those beyond.
Secondly, we see that the arrangement of the atoms in general does not
follow the usual rules of valency, but that each atom is combined with a
much larger number of atoms than corresponds to its normal valence.
In the past it has been customary to consider that solids and liquids
are held together by the “forces of cohesion” and to call these “physical
forces” as distinguished from chemical forces. Often these distinctions
are known to be rather vague, but in the more recent years, with growing
confidence in the atomic theory, we have been accustomed to consider
that chemical phenomena are those involving an alteration of the structure
of molecules, while physical phenomena are those in which only the mole-
cule as a whole is concerned. This distinction throws the whole difficulty
back on to the definition of the molecule. In gases there is usually no
uncertainty as to the size of the molecules, but in liquids and solids no
really satisfactory methods have been found for determining molecular
weights.
-1s
long as we cannot definitely determine the molecular weights,
it thus remains impossible to distinguish sharply between chemical and
physical phenomena. Nevertheless, much discussion has arisen of late
years over such questions as whether adsorption and surface tension are
chemical or physical phenomena. The overwhelming consensus of opinion
seems to be that these are both physical phenomena.
In the following pages the writer hopes to show that there is no present
justification for this distinction between chemical and physical forces.
Cohesion, adsorption and surface tension are all manifestations of forces
similar in their nature to those acting between the atoms of solid bodies.
It
is therefore advantageous
to
look upon these forces as direct results of
chemical affinity. In this way it becomes possible to correlate these
so-
called physical phenomena with the known chemical characteristics of the
atoms and groups of atoms forming the bodies.
Theories
of
Chemical Constitution.-The fact that each atom in a
crystal is usually combined with
a
larger number of adjacent atoms than
corresponds to its valence, is not in conflict with recent theories of the
constitution of chemical substances.

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