1. A. CAMPBELL Oberlin College, Oberlin, 0 h i o
Tmm ARE MANY attempts at present to introduce the concept of spatial configuration of molecules into the introductory chemistry course. Stick and ball models, Fisher-Hirschfelder flattened spheres, plastic discs, marbles, ball bearings, and wooden or plastic spheres have all been used in building up representations of molecular shape, and of crystal structure. Every representation is inadequate from some standpoints, though sufficient from others. Since atoms are not hard, solid spheres, but are space filled with fluctuating electric fields it is fairly safe to say that no model will ever suffice to reproduce all the characteristics of atomic and molecular behavior. Nor is such complete representation necessary. A model which allows interpretation of most of the chemical and physical properties of a substance in terms of its geometry is often sufficient. This model must also have the property of being clearly visible and understandable when viewed from a distance if it is to be useful in classwork. CONSTRUCTION OF MODELS
Molecules interact primarily through their outel electrons which also determine the van der Waals radii, or, in electrovalent substances, the ionic radii. Thus models based on these radii are to be preferred to those using spheres of indeterminate radius or of covalent bond radius. The usual objection which is raised to these van der Waals models-such as the FisherHirschfelder type-is that they are hard to see because of the compactness of the aggregate and because of the small size of the models currently produced. In an earlier paperZ we presented a periodic table based on atomic models on the scale of one inch to the Angstrom. This is a particularly convenient scale to work with since distances in Angstroms may be given to the shop worker without the use of a conversion factor, and since the range in atomic size on this scale is from about one to five inches in diameter, which gives good visibility even at the distances involved in the largest classrooms. In this paper we are extending this scale to molecular and crvstal models. as shown in the fimres. These molec&r models are based on experimental internuclear distances and angles within the molecule, and van der Waals radii or ionic radii for the exterior surfaces as in the Fisher-Hirschfelder system. Admittedly the van der Waals radii are not known with urecision. hnt.
USE OF MODELS
Presented before the Division of Chemical Education at the 112th mceting of the American Chemical Society in New York, September 1519, 1947. * CIMPBICLL, J. A,, 3. CHEM.EDUC. 23,525 (1946).
One of the most illuminating uses results from arranging the models of the molecules of the elements according to their position in the periodic table as in
u
Figure 1.
Molecular Models of the Nonmetallic Elements in the U p ~ e rRight Port+ of the Periodic Table.
the relative sizes are adequately represented in the present tabulations. The models are permanently assembled rather than interchangeable so that variation in angle and bond distances in a sequence such as the oxides of nitrogen may be noted. Where internal rotation or bond breaking is present in the natural material the models also rotate (as in hydrogen peroxide, Figure 2) or break open (as in sulfur, Figure 1). Each atom is painted for ease in identification. The color denotes the family or column in the periodic table (black, carbon; blue, nitrogen; red, oxygen; grcen, fluorine; orange, hydrogen) and the hue the particular element. Thus fluorine is dark green and chlorine broGine, and iodine each successively lighter shades of grem.
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APRIL, 1948
Figure 1. It should be remembered that the models represent the molecular state which is stable in the range of moderate temperatures of primary interest to the beginning chemist. Si has the same crystal structure as diamond hence we use the diamond model here t o represent Si and the graphite for carbon. Unfortunately, we have not as yet completed models of all elementary structures;but even vith the few models here illustrated many of the important structural principles may be demonstrated. The change from diatomic to polyatomic molecules is clearly s h o ~ m and , the trend from polyatomic back t,o monat,omic may be easily deduced. In the halogen family and hydrogen each element shares an electron pair between two atoms giving a coordinat.ion number of one. In oxygen and nitrogen the coordination number is also one. This is unusual since oxygen has available sufficient orbitals to coordinate two atoms into a ring or chain structure a s does sulfur, and nitrogen could coordinate three to give puckered planes or closed tetrahedra as does phosphorus. These diatomic molecules with accompanying multiple bonds are ordinarily iuterpret,ed in terms of the small size of these elements along with their very large tendency to complet,e, or nearly complete, an octet. The larger sizes of phosphorus and sulfur allox the greater coordination numbers and a closer approach to single bonding. As the size increases the tendency to form covalent honds decrease?. Thus oxygen exists as a
d F i g u ~ e2. Models of Some Nonmetallis Oxides
KaO
NO NO,
NO.-
HgOz
SO.
H~O,
so8
Piplebonding, while sulfur has an eight-membered rinx, and selenium and tellurium infinite chains-all essent&lly single bonded. In the carbon family the rhange from multiple bonding in graphite, through the singly bonded tetrahedral dicon and germanium, to t,hr eight-coordinated tin, and the twelvr-coordinntrd lead illustrates to an even greater
Fi(rure3. Crystal ModslsofCarbon. Graphitelsat Left end Diamond et Right.
extent the decreasing tendency to form a few, strong covalent bonds as the elements become more metallic. The decreasing ability t o form multiple bonds as the number of available orbitals diminishes is.well illustrated by t,he lengthening of the bond in moving along the row in the periodic table from nitrogen to oxygen and from oxygen to fluorine. Carbon as diamond or graphite (Figure 3) is interesting for it is here that the maximum number of strong covalent bonds can form since four electrons and four orbitals are available for bonding in each atom. Even here, horerer, multip1icit.y of bonding is attained by forming the planar, three-coordinated structure of graphite in xvhich the bonds averaze - one-third double a n j tn-o-thirds single in character. The approximate maintenanre of this equality of number of available electrons and empty orbitals throughout Groups 4', 5', 6', and 7' as \\-ell as the transitional 8', 9', and 10' accounts, of course, for the hardness of these elements. It should be not,ed t,hat carbon is the only element of this group ~rhichshovs the multiple bonding of the graphite t,ype of structure. This is again consistent mith the trends mentioned in the nitrogen and oxygen families. The models in Figure 3 demonstrate clearly the difference in densit,y of diamond (3.51) and graphite (2.25) since each model contains the same number of atoms. In any plane t.he graphitic carbon nuclei are closer than the diamond carhon nuclei, but the big interplanar distance in g r a ~ h i t eorerhalances this effect and decreas~s the dens& In Figure 4 the diamond crystal is expanded, or exploded, to s h o ~ rthe T-ariousvays in which the atoms pack togethrr. One can pick out the tetrahedral arrangement, the cvclohexane puckered rine. and the -, "straight chain" of organic chemistry mith no difficulty. It also becomes clear +-hen one looks at the faces exposed n-hy 17-e get t:he geometrical term "diamond" from these crvetals. STRUCTURE IN OXIDE SEQUENCES
The structural arrangements in the oxides of nitrogen, hydrogrn, phospho~.us,and, sulfur are shon-n in Figure 2 . S S O and S O are linear sincc all electrons
JOURNAL O f CHEMICAL EDUCATION
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..-. A.
,
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1:
ure 2). This leaves a free pair of electrons on each of the apical phosphorus atoms so that four more oxygen at,oms can add to give the highly symmetrical P4010 (Fim~rc5) molecule r..lther tl1:111 IIIC very ~~niyniui~trirnl 0 -\/-\*/-
0
B
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rigule 4. - ~ ~ ~ ~ ~ ~ d ~i cryst.l d . .~ showing ~ from~~ . f tto~ Right: (1) Ths Nsopentane Tetrahodrd Carbon Arrangement; (2) the "Puckered" Cyclo-herano Ring (Chai- Form): (31 thm "Straight
chain"carbon~
.
~
~
~
~
~
~
are engaged in bonding except some on the terminal atoms. NO,, on the other hand, is bent, showing that there is a t least one electron on the central nitrogen which is not engaged in bond formation. The planarity of the NO3- again indicates that all the nitrogen electrons are bonding. Similar considerations account for the changes in the sulfur series. SO2shows the presence of a free electron pair on the sulfur by its angular geometry. In SO1 this formerly free pair is bonding the additional oxygen, while the addition of a fourth oxygen plus an additional pair of electrons gives the tetrahedral structure of the SOC. It is apparent from the geometry of H20, HzOz,and P40sthat here again there are nonhonding electrons on the oxygen and phosphorus atoms. The existence of the various gaseous oxides of nitrogen is further evidence of the great tendency of these small atoms to form multiple covalent bonds.
"0
formerly required of all students of chemistry. This would seem a good place to question the advisability of d requiring a beginning student t o write structural formulas for nonexistent molecules, or for molecules which exist structure is not known by the ~ but~whose t correct . instructor. The electronic structure of the oxveen - molecule is an outstanding case in point. Further correlations of geometry with chemical properties may be illustrated by showing that the slow hydrolysis of COz compared to that of SOz may be interpreted. Thus SO2is already a bent molecule and need only add a molecule of water to the sulfur, whereas CO* is linear and must undergo considerable deformation before it can form the stable hydration product, GO3- that has the same configuration as NO3-. The ability of NHs (Figure 6) to form complexes, though NH,+ (Figure 5) cannot, is easily interpreted in terms of the molecular structures. The pyramidal shape of the ammonia molecule shows the presence of a free pair of electrons at the apex of the pyramid. In the ammonium ion these electrons are engaged in honding the fourth hydrogen and so are not available for complex formation. The transition here, as that in the phosphorus oxides, is demonstrated by adding "caps" of the Droner size over the ~ositionsoccu~iedhv the free electron pair and thus converting the simpler model to that of the more fully bonded molecule.
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STRUCTURE IN CHEMICAL REACTIONS
The use of such models in discl,ssing chemical reactions is well illustrated by the oxidation of phosphorus by oxygen. I n the first step the phosphorus tetrahedron, Pa in Figure 1, is expanded and an oxygen atom introduced between each phosphorus to give P,Os (Fig-
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0
1
STRUCTURE IN CRYSTALS
The distinction between simple molecular, ionic, and
-giant or macromolecular tvDes of crvstal is di5cult for
the student to make wheniooking at the usunl kind of crystal model built oi spheres. S I I Cdifierenriation :~ is much easier when the models are presented as in these fieures. In the usual model for the ionic t v ~ of e crvstal. u s h e taneent s~heres.the s~hericalsvmmetrv of the ionic particles is stressed, while here in the macromolecular (diamond, graphite, etc.) type (Figure 3) the directional bond and nonsymmetry of the atom is pointed out. The crystal made up by packing of simple molecules is easily visualized in terms of these two extreme types. Apparent anomalies such as aluminum chloride (Figure 5) are readily interpreted in terms of electronic strnotures and the corresponding low boiling and melting points explained. The use of these molecular models in inorganic chemistry makes it easier to show the student that there are relatively few simple molecule^,"^ and to interpret the reason for this in terms of possible crystal types and varying chemical bonding. a RAKESTRAW, N. W., J . CAEM.EDUC. 23,417 (1946).
I1 - -
Figvre 5. Models of (Left t o Right) Aluminum Chloride (A12Cls), Phosphorus "Pentoxide" (Pr0~o).and Ammonium Ion
(NHI-1.
-
" &
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APRIL, 1948 STRUCTURE AND PHYSICAL PROPERTlES
Several examples of interpretation of physical properties in terms of structure have already been mentioned. Another of interest is the transition in the viscosity of liquid sulfur upon heating. With the pegged model of the eight-membered ring (Figure 1) it is possible to break the ring a t any point by applying a little energy, i. e., by thermal agitation, thus converting ring to chain. The student haslittle di5culty realizing that tangled chains would resist flow more than compact rings. Upon further agitation, i. e., raising the temperature, the chain degrades into shorter segments thus accounting for the experimentally observed subsequent decrease in viscosity. 'The variation in acid strengtb of the simple hydrogen acids may be well correlated with the aid of Figure 6. In the horizontal rows the acid strength increases from left to right since the oxidation number of the "central atom," i. e., the nonhydrogen atom, is decreasing and thus the attraction for a hydrogen is decreasing. In the vertical rows the acid strength inkeases from top to bottom since the oxidation number is constant but the size of the "central atom" is increasing, thus diffusing the effect of the negative oxidation number over e larger volume and lessening the attraction for a hydrogen in any one direction. Thus, in this set, NH, is the weakest acid (the strongest base) and HI is the strongest acid (the weakest base). Numerous other correlations of properties might be Molecular Models of Hydrogen Acids: NHI: H1O. HP. HC1. HBn HI. mentioned. Since these are all available in numerous books on structural chemistry's 5 . there seems small gain in listing them here. However, structural chemis- try involves such simple pictures and allom such widely applicable correlations that it is indeed unfortunate not ' PAWLING, L., "Nature of the Chemical Bond," Coraell Univ- to make wider use of it in the elementary approach to ersity Press, 1945. chemistry. The concept that an atom or molecule has 6 WELLS, A. F., " S t r u ~ t ~ mInorganic l Chemistry," Oxford, size and shape is a t least as important as that it has Clarendon Press, 1945. 8 YOST.D. M.. AND H. RUSSELL. "Systenmtic Inorganic Chem- weight, and is much more useful in the interpretation of most chemical phenomena. istry," ~mntice-~sll, 1944. .
