James B. Pierce'
Case Institute of Technology Cleveland 6, Ohio
I
Molecular Models: A general chemistry exercise
A section of 25 freshmen was given a list sf bond angles, covalent radii, and van der Waals radii for a number of simple structures. A sufficient number of foamed polystyrene spheres (1'/2 in. diameter) was provided for each student to build a~modelof one molecule or ion. The following suggestions were made: (1) for assembly of atoms use water-based glue or pipe cleaners; (2) for coloring agent use water color or enamels formulated with aliphatic solvents; (3) use Kenney's2 method for reducing the spheres to the desired size. The students were left to devise their own methods for locating bond BxeE and "covalent faces." A total of 32 models was constructed as shown in the photograph. After having built these models, the students were quite receptive to the ideas associated with electron pairing, orbital overlap, and hybrid bonding. Hybrid bonds were explained as a natural result of the mutual repulsions between the bonding orbitals. The concept of lone pair electronsa was introduced to explain bond angles in H20, NHa, etc. With the first hand knowl-
edge that SOa=, CClr, and NH4+ all have the same geometry, the idea that ions are, in essence, molecules hearing formal charges opened the students' minds to the fact that ions have characteristic properties. Other concepts such as acid-base phenomena, resonance, dipole moments, hydrogen bonding, etc., were accepted by this class much more readily than by previous classes. One unexpected dividend was an enthusiastic interest in the laboratory work. Student models.
Present address, Department of Chemistry, Lowell Technological Institute, Lowell, Massachusetts. 35.513 (1958). KENNEY, MALCOLM E., J. CHEM.EDUC., a CARTMELL, E., AND FOWLES, G. W. A,, "Valency and Molecular Structure," Academic Press, Ine., New York, 1956, p. 169
Volume 36, Number 12, December 1959
/
595
