A. Harris Stone Southern Connecticut State College, New Haven and Irwin Siegelman Silver Burdett CO. Morristown, New Jersey
An Atomic and Molecular
I
Orbital Models Kit
Thc study of the nature of the covalent bond in terms of partial ovcrlapping of electron probability distributions, orbital hybridization, and subsequent considerations of n~oleculargeometry is no longer confined to college-level chemistry. Modenl programs for high school chemistry treat these concepts in qualitative detail. The use of models to illustrate and clarify difficult physicochemical concepts has become essential for the effective teaching and lcarning of such concepts.
Model Construction
Each tnodule of our kit mas nladc by standard plastic vacmnn-forming tcch~liques.~Originally wooden plunge forlns were used. These proved inadequate sincc "lnark off" of thc wood grain in the hot plastic resulted. Cnrrently almnu~ummolds with a highly polishcd contact surface are used to fashion the methyl inethacrylate plastic modules. The n~odulesfor s and p nonbonding orbitals are made in halves which are then cemcnted together with niethylene dichloride. The niodules for s and p bonding orbitals are similarly fashioncd. The latter, however, involve the inclusion of thc visible overlap. This is accomplished by cementing a spherical segment within onc bonding modulc and cutting out a nlatching srction of another bonding modulc. Fitting the two bonding modules together leads to the visible overlap. Figure 1 shows the senaratcd bondinsz s modules and their fit to deuict the hydrogrn n~olcculewith the s-s bond visible. u
Figure 1.
HZ before and after orrembly.
Among the most difficult conccpts of modern atomic theory for students to comprehend is the statistical model of electrons in atoms,' and the subsequent treatment of the covalent bond by valence bond t h ~ o r y . ~ I n developing models to assist teacher and student alike in their considerations of the valence bond theory of the nature of thc covalent bond (i.e., the overlap of atomic orbital probability distributions) our major objective was to provide an unhampered view of the interior structure of covalent n~olrcules. I n the present models, the overlap which constitutes the covalent bond can be seen. While inany previously described models depect atomic orbital probability distributions or molecular geometry satisfactorily3 our models, we believe, illustrate both sinlultancously. I ANDEE~S D. , H., AND K o x ~ s ,It. J., "Fundamental Chemistry," John Wiley and Sons, h a . , New York, 1962, chap. 4. l A ~ ~ D.~ H., ~ AND w KOKES, ~ , R. J., chap. 6. See, for example, FOWLES, G. W. A. J. CHEM.EDUC.,32, 260 F. L., J. CHEM.F:Duc., 34, 217 (1957); and (1955); LMIBERT, HOOJENBOOM, B. E., J. CWEM. EDUC., 39,40(1962).
Figure 2 .
Hydrogen fluoride l r ~ pbonding) ond methone IS-sp3 bonding)
It is emphasized that modules of our kit are strictly qualitative representations of orbital probability distributions. Using quantitative values for the probability distributions of the 1s and 2 p orbitals, would have required a much larger and more expensive scale of construction in order to allow for mechanical attach-
* The Models Kit consiats of 46 modules, including 14 different shapes and sires from which all of the afordmentioned structures can be fahriested. The modules vary in size from &in. diameter for s orbital modules to 15-in. length of p type modulea. The model of methane as shown is approximately 24 in. in diameter. Work is presently under way to produce the kit and to offer it for sale. It is planned to be made availahle by September 15, 1964. Since production costa are not fully known at present, an exact pries is unrtvailablble. The produeen are however attempting to keep costs na low as possible to insure widest acceptance. Information is svnilable by writing directly to Logotronies, Inc., Suite 1007, 1518 Walnut Street, Philadelphia 2, Penna. Volume
41,Number 7, July 1964 / 395
