CHAPTERS in CRYSTAL CHEMISTRY for COLLEGE FRESHMEN. PART 11'" CHARLES W. STILLWELL University of Illinois, Urbana. Illinois
Chapter 11. The Structure
T
HE great variety and complexity of silicate formulas have puzzled chemists and mineralogists for many years. Perhaps one of the greatest achievements of crystal chemistry has been the systematic classification of these minerals. After many years of work on the crystal structures of a large number of silicates it has been shown that even in these complicated substances the ionic radii are of prime importance in determining the structures of the crystals. The silicate structure as pictured by Bragg may be briefly outlined as follows: 1. Oxygen ions, being the largest, form the skeletons of all silicate crystals. Their arrangement approaches that of close-packed spheres. (It is interesting to note that errors in the assignment of formulas to silicates have frequently occurred because the importance of oxygen in the formula has not been realized.) 2. The several metallic ions of the silicate are fitted into suitable gaps in the oxygen skeleton. The arrangement depends upon interatomic distances and ionic radii. The coijrdination numbers will depend upon the values of the RA:Rx ratio as already developed in Table 2 of Chapter I. For example, the radius of the silicon ion is 0.4, that of oxygen is 1.40, Rsi/Ro = 0.285, and the silicon ion therefore has a coordination number of four. Silicon occurs in silicates surrounded tetrahedrally by four oxygen atoms. This oxygen tetrahedron with a silicon at its center is common to all silicates. The groups may exist as individuals. or adiacent tetrahedra mav share corners or edges, but rarely faces. The experimental evidence1 indicates that the Si-0 bond is not a true ionic bond. The silicon and oxygen atoms have only been partially ionized, and the bond might therefore be classified as "polarized ionic." The coordination numbers (for silicates, the number of surrounding oxygen atoms) of the other elements found in silicates may usually be calculated from their ionic radii, and the spacial arrangement of the ions is as indicated in Table 2 (Part I). Thus, eight oxveen ', atoms at the corners of a cube will surround
d
* Presented before the Division of Chemical Education at the Washington meeting of the A. C. S., March 29, 1933. :----t Part I appeared in the BRAGG.W. L.,Tram. i
of
the Silicates
every sodium ion. The bonds between metallic ions and oxygen are essentially ionic. 3. The oxygen atoms are shared by the metallic elements; in fact, the silicon-oxygen tetrahedra (either individuals, or groups of tetrahedra) are connected with other like groups by the metallic ions. 4. In keeping with the principles of isomorphism already developed, the oxygen (ionic radius = 1.35 A.U.) may be replaced by a fluoride ion (1.33 A.U.) or an hydroxyl ion (1.35 A.U.). 5. The cations tend to be distributed symmetrically throughout the crystal, since this affords the greatest stability. Pauling has reached the same general conclusions with regard to the structure of silicates as have the Braggs, but his mechanical picture of such crystals is somewhat different. He pictures as the "building blocks" polyhedral units (tetrahedra, octahedra, or cubes) sharing comers, edges and faces, as contrasted with Bragg's oxygen skeleton. Both account satisfactorily for the same structures. With this general picture, it has been possible to place all the silicates whose structures are known into a very few groups based on the silicon-oxygen arrangement. 1. Self-containedgroups Si04-orthosilicatesExamples : Garnet, (Fe,Mn)sA12(Si04)s; Olivine, Mg2SiOl Single tetrahedra joined by metal ions as in --,A.
S*lLS.
Si20i--Examples: Sc2Sie07,CazZnSiz07 Two tetrahedra sharing one oxygen-joined to other SizO, groups by metal ions SbOs-Examples: BaTiSisOo Three tetrahedra sharing edges with each other joined to other Si00groups by metal ions. 2. Silicon-Ox~genchains A. The pyoxenes (SiOs group)-Example: Diopside, CaMg(SiOa)~ Simple chains of Si-0 tetrahedra, each sharing two oxygen atoms (Figure 6A) B. Double chains (Figure 6B) The am~hiboles (SiaOll eroun)-Examnle:
In this type of crystal the metallic ions form the links between parallel chains. The binding force between chains is ionic and is not as strong as the silicon-oxygen bonds within a chain. This gives rise to the well-known fibrous nature of the amphiboles.
0 oxygen
FIGURE ~.-&LIcoN-OXYGEN SHEETS The micas. Example: Muscovite, H1(K.Na)AI&Oa,
A-Single chains Thc pyroxenes (SO, group). Eaample: Diopddc. Cahlg(SiO,),. B-Double chains. The amphiboles (Si40Llgroup). Example: Asbestos, Ca,Mg@irOn)l(OH). (tremolite).
This general picture of the silicate structure has an interesting bearing upon the position of silicon in the periodic table. Considering its neighbors, we find that Mg and Al form continuous ionic lattices. P, S, and C1 form structural groups which act as single ions and these complex ions combine with metals in ionic lattices. Si forms SiO4 groups which may share oxygen atoms and be l i k e d together to form Si-0 complexes with indefinite extension in space, giving rise to a variety of structures.
3. Silicon-oxygen sheets Examples: Biotite, (H,K)2(Mg,Fe)2(Al,Fe)r(SiOSr ; The micas, e. g., Muscovite, HZ(K,Na)A4SiaOm If the double chain is extended (as in Figure 7) a sheet is formed. The metallic ians furnish the ionic forces between sheets, weaker than the silicon-oxygen bond within sheets, hence the minerals tend to cleave in the familiar thin layers.
4. Three-dimensional silicon-oxygen network. An extension of the silicon-oxygen sheets. The metallic ions are distributed throughout the network, and may be replaced by other metallic ions without disturbing the network. This accounts for the peculiar properties of the zeolites, minerals of this type. As is commonly known these zeolites are very useful for softening water because the Ca++ and M g f t ians of the water are able to replace the Nat ion in the zeolite and are thereby removed from the water. Here again, the facility with which various metallic ions may be introduced into the zeolites depends, among other things, upon their sizes, and this suggests a picture of metallic ions weaving their way through the silicon-oxygen network. The several forms of SiO? are also snecial networks of siliconoxygen tetrahedra. One of these, cristobalite, is shown in Figure 8.
FIGURE8.-CRISTOBALITE ( S I ~ SSTRUCTWER ) Showing characteristic Si-0 tetrahedra.
Finally, silicon is to inorganic chemistry what carbon is to organic chemistry because they both can form chains--carbon by combining with itself (-C-C-C-C-) and silicon through oxygen linkages (-O--Si-OSi+Si-)
.
Chapter III. ,Jfetalsand Alloys. We are in the habit of thinking of three states of matter-gaseous, liquid, and solid. It seems almost necessary to add to these a fourth-the metallic state. To even the casual observer the properties of metals diier so markedly from those of other solids that it seems as though there must be some fundamental diierence in the way the atoms are put together. The nature of metals has evaded description until very recently, but now we have a rather clear picture of the metals even though it is by no means complete in every detail. The characteristics of metallic elements may be most clearly understood by comparing their structures with those of the non-metallic elements. In Table 12 the elements are grouped into four classes, according to their crystal structures.
The Jfefallic State
This results in a sort of layer lattice, the coordination number within a single layer being 3. In selenium we find the atoms attracted toward each other so that each atom has two near neighbors, forming long spiral chains. Finally, in iodine we have a typical molecular lattice groups occupy points in the crystal lattice. in which 1% These structures take on real significance if we consider the valence of the ions of these elements and assume that the atoms are held together by homopolar (electron-sharing) forces. It is evident that in a group of two iodine atoms the octets are complete and the xx
group is therefore stable
..
I ? I :. In crystalline xx .. iodine, then, within a group 1% the forces are homopolar, while the groups are held together by weak molecular forces. The electron distribution in a ?;
TABLE 12
CRYSTAL STRUCTURE OP TAE ELEMENTS
(~arths) Class I
Those in Class I are the true metals. They are either close-packed hexagonal (C.N. = 12), facecentered cubic (C.N. = 12), or body-centered cubic (C.N. = 8). The first two types of packing are illustrated in F i m e 9. In the true metals, then, the coordination numbers are high. The crystal is composed of approximately spherical units packed as close together as possible. In Class I1 are elements which still have the three typical metallic structures, slightly modified. T h u s , cadmium and zinc are closeC;g-p& ' Cubic packing Closepacked hexagonal, but the distances between atoms Frome 9 along oneaxis is greater than when spheres are packed as closely as possible. Aluminum and lead are cubic, but the distances between atoms indicate that the spheres are not as close together as they might be. Class I11 is perhaps the most interesting group. Let us consider four typical examples, diamond (C, Si, Ge, grey Sn), As (Sh,Bi), Se(Te), and If. In diamond each atom of carbon is surrounded tetrahedrally by four other atoms, the whole lattice being cubic (ZnS in Figure 2, Part I): In arsenic we find that each atom has three near neighhors in two directions, while in the third direction the neighhors are farther away.
I
Class 11
I
class I n x x
..
0 0
;Se ? Se * e t c ., selenium chain is as follows, " Se x x .. 0 0
each Se having six electrons. Again, then, electron sharing holds the Se atoms in an endless chain while the force between chains is a metallic or molecular force. In As, each atom may hold three neighbors by the sharing of electrons, hence the layers of atoms are held by homopolar forces and the forces between layers are metallic (defined below) or molecular. The structures of selenium and arsenic may be conceived as distorted simple cubic and the coordination is six in each case, although as pointed out the six atoms are not symmetrically located. Finally, it is possible for C to hold four neighhors by sharing of electrons and therefore in diamond the crystal is homopolar throughout, and is equally strong in all directions. To summarize, when an atom has 8-N valence electrons i t can hold N neighbors by sharing electrons (where N = 0 to 4). Now in passing to Class I1 and Class I elements two marked changes occur. The number of neighbors of each atom increases beyond four, and i t is obviously impossible for the atoms (in Zn, for example) to be held together by sharing one electron with each of 8-N neighbors because there are not enough electrons available. Here we probably have the underlying cause of the metallic bond. If there are not enough electrons available for sharing, the need arises for a new kind of bond in which an electron can serve for more than two atoms, and so the metallic linkage comes into
being. Probably the metal atoms lose their valence electrons and become positively charged ions, while the detached valence electrons are free to circulate around to some extent. We have a picture of positive ions held together by a sort of gas of negative electricity. The properties of a true metal are better understood from this picture. Metals are good conductors of heat and electricity (because the unattached electrons are free to carry current). They are opaque.' They are ductile and malleable,+ rather than brittle, due in some way to this particular type of metallic binding force. If the properties of the elements in any horizontal period of the periodic table are compared it may be seen that the so-called metallic characteristics decrease as the element becomes less metallic in structure. For example, conductivity decreases and brittleness increases in going from Cu, Zn, Ga, Ge.** The case of tin is interesting. . I t is one of the borderlie elements between Classes I1 and 111. There are two forms of tin, white and grey. The white tin is an elongated cube (tetragonal) and the spheres are therefore not as close-packed as in true metals. Nevertheless it has some metallic properties, although it is a comparatively poor conductor of electricity and is quite brittle. Grey tin has the diamond structure, in which the atoms are held by homopolar rather than metallic forces, and so its properties are definitely nonmetallic. Decided dilTerences in properties between elements in different classes (e. g., Cu and As) are to be expected. There are also interesting variations among the true metals of Class I.
The Class I metals end at the vertical line drawn toward the right of the chart. One would expect the con-
CONDUCTIVITIES AND MELTING POINTS OF THE METALS
In Figure 10 (lower graph) are plotted the atomic conductivitiesf of the metals of Class I and Class 11.
ductivity of metals to increase with the mobility of the electrons which carry the current. This is evidently * The color of crystals is due t o the effect of electrons vibrat- true, and is the predominant intluence, since the chart ing within them. Thus, a blue crystal appears blue because the shows unmistakably that the univalent metals are vibration of the electrons within i t is such that all the wave-lengths the best conductors, and the conductivity decreases except blue, or of light complimentary to blue, are absorbed sharply for the divalent metals and still further for the passing through the crystal. In a black substance such as charcoal the electrons are held with varying degrees of firmness trivalent. It is of interest to note that the tetravalent and so all light Is absorbed. I n metals, the electrons absorb metals (Ti, Zr, Hf) are very poor conductors and all frequencies, but they immediately radiate them all again. actually behave much like homopolar structures. This gives rise t o the familiar metallic luster. Graphite is an Though they have close-packed hexagonal lattices and unusual case. It forms a layer lattice, and each carbon atom has three neighbors within a given layer. But this leaves one are classed with the true metals, they still retain a electron on each atom unshared, and it is this more or Less strong resemblance to the other members (C, Si, Ge) free electron which gives graphite its metallic properties. t The ductility of metals is related to their closepacked struc- of Group IV. The "ups and downs" in conductivity twes. The close-packing affords the largest number of "planes from vanadium (Cb, Ta) through nickel (Pd, Pt) of flow" for the crystals. This is evidenced by the fact that facecentered cubic metals (which have the greater number of possible are due to the variable number of electrons in the outer planes) are more ductile than hexagonal metals, even though they shell of these transition elements (as evidenced by their are both close-packed with C.N. = 12. variable valences). When we pass over the line from ** Conductivity increases again for arsenic and selenium, and these elements exhibit metallic luster. For this reason i t is as- copper (Ag, Au) to zinc (Cd, Hg) we enter Class I1 sumed by many that the forces between layers in arsenic and in which the structures are distorted and so the conbetween chains in selenium must be metallic. ductivities cannot be explained as well. 1 The specific conductivities of metals are not very useful for cornparisoh, since they involve equal volumes rather than equal Aluminum stands out on the chart as an exception numbers of atoms. For the derivation of the term plotted in to the rreneral trend. Its conductivity is actually atomic conductivity Figure 10, namely, see Hume-Rothery, geater Shan that of magnesium. A study of other (atomic volume) 213 properties of aluminum leads to the suspicion that it lac. cit., p. 4.
is only partially ionized in the metallic state and therefore exists as a univalent ion. If few electrons, weakly held, make for maximum conductivity in metals, they would also be expected to produce the weakest crystals. It is interesting, therefore, to consider the melting points of the metals as plotted in Figure 10 (upper graph). Although there are intermediate variations which are not explained by our simple picture of the metal, in general the stronger metals are the poorest conductors of electricity. Aluminum is again the exception. We have seen that the conductivity of aluminum is greater than would be expected, and it follows that aluminum should be abnormally weak and have a comparatively low melting point. This is confirmed by the position of aluminum in the chart. These relations help to clarify our picture of the metallic state-positive ions, held together by electrons more or less free to wander about among those ions. I t is apparent that those metals which possess the smallest number of loosely held electrons are the best conductors of electricity and build the weakest crystals. We are accustomed to consider the hardness or tensile strength of a crystal as a measure of its strength and one would therefore expect these properties of metals to vary in the same regular manner as do the melting points. There is a general agreement, in that the lowmelting metals are soft and exhibit low tensile strengths. I t is not safe to carry the analogy too far, however, because tensile strength and hardness may be greatly influenced by the manner of working the metal* and it is diicnlt to obtain comparative data for metals which have all been treated the same way. THE METALLIC BOND IN IONIC COMPOUNDS
There is an interesting group composed of compounds which are rather metallic in appearance and show other metallic characteristics. These are the sulfides, arsenides, tellurides, and selenides of a number of metals. While the structures of these are not thoroughly understood, particularly as regards the type of forces which hold the units together, the properties of such crystals (opacity, electrical conductivity, ability to alloy with one of their constituents, etc.) support the idea that part of the binding forces a t least are metallic. Two of these types are NiAs or FeS in the AX group (Figure 2, Part I) and pyrite (FeS) in the AX2group (Figure 11). Let us glance a t the model for pyrite. It is evident that there is something abnormal about it. The iron atoms are in a face-centered arrangement. But the sulfurs are paired; they exist as S groups, probably as molecules. Whether the iron atoms are ionized or not is not known. At any rate it is believed the type of binding leaves some electrons free to wander and impart metallic properties to the crystals. - -
-
* The tensilestrength of cold drawn wire, for example, is always much greater than that of a casting of the same metal. See Part 3 of thie paper.
THE RELATION OF
IONIC,
HOMOPOLAR, AND METALLIC BONDS
There is an interesting similarity in these three general types of bonding which is helpful as an aid to a satisfactory understanding of their natures. The ionic bond results when a metallic atom is able to give up its electron to a non-metallic atom. The former then becomes a positive ion, the latter a negative ion and the two are held together by attraction of opposite forces. The transition from the ionic to the homopolar bond (sharing of electrons) may be viewed as an extreme case of polarization in which the positive and negative ion fields are so far distorted that they actually overlap and the two electrons are shared by the two atoms. Finally, there is a close relation between the homopolar and metallic bonds; while in the homopolar bond two electrons are shared by two atoms and are held to those atoms, the metallic bond consists in the sharing of a small number of electrons by a larger number of ions. This makes it necessary for the electrons to circulate from ion to ion. The properties characteristic of these several types of binding are summarized in Table 3 (Part I). The intermediate types listed there may readily be understood if the three main types are clearly in mind. ALLOYS
Civilizationhas passed through a Stone Age, a Bronze Age, and an Iron Age. Many would say that we are at present entering an Age of Alloys. As we learn more about them, alloys have become more and more valuable in industry. Here again, however, it is only recently that we have begun to understand the fuudamental nature of alloys. Such understanding has already been of great help in increasing the number of useful alloys available. When two (or more) metals are melted together and allowed to solidify, one of three things may happen.
1. The two may crystallize independently. The alloy will consist of an intimate mixture of the two crystal forms and is not homogeneous even though it may appear so. I t s properties are apt to be the average of those of the two constituents. 2. They may form solid solutions. 3. They may form new, so-called intermetallic compounds. THE NATURE OF SOLID SOLUTIONS
as this close-packing becomes distorted (as in Zn and Cd, for example) the metallic properties become weaker. That is, the elements become (1) harder and more brittle, (2) less malleable and ductile, and (3) poorer conductors of electricity and heat. It is easy, then, to understand in a general way the properties of solid solutions, since they will depend on the distortion of the pure solvent lattice by the solute atoms. Solid solutions are harder and more brittle than pure metals. (Pure copper is harder than pure tin, but 5 per cent. of tin added to copper gives an alloy twice as hard as pure copper.) Their electrical resistance is greater. When the solute is a true metal the change of properties is not marked, since a true metal solute does not cause much distortion in a true metal solvent lattice. Very marked alteration of properties may result as the solute becomes less metallic, however. Many interesting applications of this principle may be recalled. Cadmium is added to copper to harden it, but does not seriously decrease its conductivity. A trace of arsenic (a "Class 111" structure) in copper, however, decreases its conductivity over 15 per cent. and is very objectionable. Solder (50 Pb-50 Sn) is harder than pure lead, but a trace of arsenic in lead shot hardens the lead even more effectively than does tin. It should be noted that when a true metal is alloyed with a non-metal, the properties of the resulting solid solution are more metallic than those of the non-metal.*
A solid solution is homogeneous and its composition may vary within certain limits. It is like any solution in that respect. Let us consider it from the standpoint of crystal structure however. Nickel may be dissolved in copper, forming a solid solution. During this process the nickel atoms replace copper atoms in the face-centered cubic copper lattice. Every time a nickel atom is substituted for a copper atom the lattice shrinks a little (see atomic radii) and this change in size may be detected by X-ray analysis. As the percentage of nickel increases, there comes a time when the copper lattice cannot accept any more nickel. It is convenient to say that a t this point copper (the solvent) is saturated with nickel (the solute). If any more nickel is added the Crystal structure changes to that of nickel, in which some of the lattice points are occupied by copper atoms. At a given concentration, then, we pass from a solid solution of nickel in copper to a solid solution of copper in nickel; marked by a change in the crystal structure from that of copper INTERMETALLIC COMPOUNDS to that of nickel. It so happens that both copper and If zinc is added to copper, the zinc atoms replace nickel have the same structure and therefore no sharp copper atoms in the face-centered cubic copper lattice. change occurs. The two are soluble in all proportions. When 32 per cent. zinc has been added, the copper latThis is called a continuous series of solid solutions. If tice is saturated and the addition of more zinc produces nickel (face-centered cubic) is dissolved in chromium (body-centered cubic), when 37 per cent. of the chro- a new lattice which i s not the hexagonal lattice o j zinc. mium lattice points have been filled with nickel atoms, I t is a structure diering from both zinc and copper the chromium lattice becomes saturated and the addi- and must therefore be due to a new compound formed tion of more nickel causes it to change to the face- between zinc and copper. In some alloys there may be centered nickel lattice in which 63 per cent. of the only one such compound. In brass there are actually points are occupied by chromium atoms. This is called three such metallic compounds, known as P-, y- and a discontinuous series of solid solutions. the two metals r-brass. Figure 12 shows the relation of these comagain being soluble in all proportions. The substitution of either larger or smaller atoms in a metallic lattice naturally causes distortion of the 4 8tr Y Y f € € lattice. When this distortion becomes so great that the I I I 1 I lattice is no longer stable, the solid solution is saturated 0% 20 30 40 60 70 80 and the addition of more solute will cause the lattice Zinc cu to chanee. Flouns 12 " There is an indication that the more non-metallic in The Tbrass nature the structure of the solute, the more it will dis- pounds, to the com~ositionof the *OY. tort the lattice of a truly metallic solvent and the less 1s a solid solution of copper in zinc; or-brass is a solid soluble it will be in the metal. Thus, copper (a true zinc in metal) may be substituted in the nickel lattice without * Attention should be called to an interesting group of binary much distortion, while arsenic substituted in the same alloys, those of the transition elements, with carbon, oxygen, and nitrogen. The resulting alloy-f iron and carbon, for lattice would cause a great deal of distortion. exampltare metallic in nature but the solid solutions fonned It should be recalled that those elements which are are of a different nature than those already discussed. They described as interstitial. The solute atoms do not replace most truly metallic are those whose structures resemble are the solvent atoms, but they fit into the empty spaces hetween most accurately the close-packing of spheres, and that the atoms of the solvent.
sen
As shown in the diagram there are ranges of composition in which two phases are mixed together; thus as zinc is added to copper, a t about 32 per cent. zinc the copper is saturated with zinc and the &brass appears along with the a-brass. Not until 47 per cent. zinc is reached does the a-brass disappear entirely. These three compounds of copper and zinc (8-brass = CuZn, y-brass = CusZns, s-brass = CuZns) are typical of a large number of intermetallic compounds. In spite of their relative importance they are rarely mentioned in a beginning course in chemistry because, as may be seen, they have very unnatural looking formulas-unnatural from the valence standpoint. With the help of information we have accumulated in the course of our studies of crystal chemistry, however, we can a t least develop a general idea as to the nature of these compounds. In the first place, if their properties resemble those of metals, we should not expect them to have formulas which satisfy the usual valences of the metallic ions, for we now know that in the metallic state atoms are not held together by the transfer of electrons. This a t once clears away a great stumbling block in the interpretation of these compounds, and we may investigate them further without worrying about valence. When we do this, however, we find that valence apparently does play a part in the formation of certain intermetallic compounds. A large number of alloys contain a compound corresponding to the y-brass. It is always cubic and is characterized by its large unit cell, containing from 52 to 416 atoms. A few of these compounds are listed in Table 13. It will be noticed that although there is no apparent regularity in the formulas, actually the ratio of the number of atoms to the number of valence electrons is constant throughout and is as 13:21.
in some degree homopolar rather than metallic, this should he evidenced by their properties. As a matter of fact, we find such compounds very hard and brittle. The melting points are high, usually higher than that of either constituent. The electrical conductivity and ductility of these compounds are illustrated in the typical example of hrass shown in Figure 13.
It should be pointed out that all intermetallic compounds do not show these homopolar properties. From the figure i t may be seen that &brass, CuZn, has a high conductivity, higher even than the solid solution of zinc in copper. The structure of such compounds has not been completely explained, hut it is reasonable to believe that it is such that the forces holding the atoms together are metallic in nature, rather than homopolar. Many illustrations might he given of the very practical value of our growing understanding of the nature TABLE 13 of solid solutions and intermetallic compounds, but two ~amvior Atoms volrnrs mrrrronr must suffice. 1. Practically all the brass used in industry is abrass. It will machine well, is soft enough to he rolled or drawn, and has a desirable tensile strength. "White brass," high in zinc, is used only occasionally for casting ornaments. In making band instruments those parts which must be shaped are or-brass. The keys, however, must he This suggests some sort of sharing of electrons be- very rigid and hard in order that their action be efficient, tween a group of atoms and reminds us of the condition and they are sometimes cast of y-brass. This introexisting in a series of homopolar crystals previously duces a complication in that the two brasses diier studied. We found that a series like SnSn, InSh, considerably in color. Of course, if the whole instruCdTe, AgI all show the same structure and similar ment is silver-plated this difficulty is overcome. properties and, in all, the ratio of atoms to valence elec2. A bearing metal must be soft in order that after trons is constant, being 2 % (See page 597.) casting it may "work in" to fit the shaft. Once having We may likewise consider a number of compounds of been fitted, however, it should resist abrasion. The different metals which have all been found to have the ideal bearing metal, therefore, is a mixture of a comhexagonal structure of e-brass. Examples are listed pound and a solid solution. Babbitt metal (90 per in Table 13 and the ratio of atoms to valence electrons cent. Sn, 8 per cent. Sb, 2 per cent. Cu) consists of the compounds SbSn and CuaSn embedded in tin. is shown to be constant. The most important features of compounds and solid If in the complicated intermetallic compounds listed above, the forces holding the atoms together are solutions are compared in Table 14.
TABLE 14 S d i d SdrUiors show properties of typical metal Cordudiuilg: High, but less than either pure connituent Hardncrr: Relatively soft, but harder than either oum conrtituent Ducldilv: Good. butlers than mure constituent Mdring Pm'nl: Generally lower than average of constituents, sometimes lower than either pare metal Tovghmrr and Snntpfh Greater than purr eonstitvmfr color: Interesting but hard to predict For example: 75 Cu-26 Niwhite 61.5 C u 4 8 . 6 Sb -"role A ; , A;. Cd-een Au. FE-blue
Inlcrncrdlic Compound, Many show homopolar properties Low-many very resistant Hard Brittle High-may be higher than either pure "Onstituent B d t t l e a o t particularly strong
BIBLIOGRAPW
No attempt has been made to compile a complete bibliography. The references given are those which have been most useful in the preparation of this paper. Several others have been selected because in general they will not he discouragingly difficult for those students who wish to investigate crystal chemistry and the X-ray method of crystal analysis in a little more detail. BERNAL,J. D., "The problem of the metallic state," Tmns. Faraday Soc., 25,367 (1929). BRAGG, W. H., "An introduction to crystal analysis;' G. Bell & Sons, Ltd., London, 1928. BRAGG,W. L., "The structure of silicates," Z. Krist., 74, 23i 1192nl ,-"-",.
BKAGG, W. L., "Atomic arrangement in the silicates," Trans. Faraday Sac., 25, 291 (1929). CLARK,G. L., "Applied X-rays," 2nd ed.,McGraw-Hill Book Co., Inc., New York City, 1932. G O L D ~ ~ ~ V. I DM., T , "Krystallbau und chemische Zusammensetzung," Ber., 60, 1263 (1927); "Crystal structure and chemical constitution," Trans. Faraday Soc., 25, 253 (1929). These two papers are essentially alike and are the main source of the material in Chapter I. HUME-ROTHERY. "The metallic state," Oxford Press, London. 1931, pp. 300-42. PAULINO, LINUS,''Ions and ionic crystals," J. Am. Chem. Soc., 49, 765 (1927). This is one of the first papers on the subject and is extremely interesting and useful. PAULING,LINUS, "Principles determining the structure of complex ionic crystals" (silicates), ibid., 51, 1010 (1929). REINMUTA, OTTO,"Some elementary principles of X-ray crystal analysis," J. CHEM.EDUC.,7, 138, 860, 1373 (1930). WARREN,B. E., "Structure of asbestos. An X-ray study," Ind. Eng. Clrem., 24, 419 (1932). G., "X-ray studies on alWESTGREN, A. F. AND PHRAGMEN, loys," Trans. F a r d a y Soc., 25, 379 (1929)-and many other papers by WESTGREN and co-workers.
WYcKoaa, R. W. G., "The structure of crystals," 2nd ed., The Chemical Catalog Co., New York City, 1931, 497 pp. The most complete simple classification of crystal structure data. "X-rays," The Encyclopedia Brittannica. 14th ed. There are many helpful diagrams of crystal lattices here. HOUR EXAMINATION IN CRYSTAL CHEMISTRY 1. I t has been found that all silicate crystals, no matter how complicated the formulas may appear, are constructed on a "framework" of oxygen ions. The arrangement of these ions forms the "skeleton" and the other ions fit into suitable gaps in the oxygen skeleton. How many oxygen ions would you expect t o find surround each of the following ions in a silicate crystal: Mg++, SibC, N a f ? I n what configuration would the oxygen ions be arranged around the Na+? Around theSi4+? (20) 7. ( a ) Classify the followings u t ~ s t a n c c i u uthe basis of the type$ of forcer which hold the bullding units in the crystal: KNOI, ZnS. SarlPh,. I'rS, I?, Cu. Give a ~ h o r o c l ~ r i r l i r property of each. (12) (b) How do you account for the fact that silver and cadmium can form two intermetallic compounds Ag5Cd8 and AgC&, in neither of which the usual valences are satisfied? (8) 3. (a) Name three properties which are more or less related to the strength of a crystal. (8) (b) List the following substances in order of their probable strength, placing the strongest &st: NaF, CaO. KBr, Tic. (12) 4. How many different crystal forms are represented in the following list? Describe any two of them. KnCuCI,.2H20, CuBr, AgBr, (NH,)~CuCl~2HzO. COO. Cd4, SrF%. (20) 5 . (a) What do you consider the chief structural characteristics of truly metallic elements) (5) (b) The diagram shows how the ductility (measured as per cent. elongation) of bronze varies with the composition.
. ~.
Compare in as many respects as possible the probable differences in structure of the alloys represented by composition A and composition B. (15)
