FRANK I. LAMBERT Occidental College, Los Angeles, California
Tm
usefulness of the molecular orhital theory in teaching organic chemistry is indicated by the number of elementary texts which now employ it. Visualization of molecular bonding has been greatly aided by drawings such as those in Noller's writing~.~JHowever, questions from students in our classes have indicated that three-dimensional models would have some advantages in atomic orbital (AO) and molecular orbital (MO) lecture presentations. Our experience with fabricating molecular models from Styrofoam suggested that this material might provide an easier approach than the use of papiermach6 described bv F o w l e ~ . ~Accordinalv, a basic I Presented as part of the Symposium on Use and Abuse of Models in Teaohing before the Division of Chemical Education at the 130th Meeting of the American Chemical Society, Atlantic City, September, 1956. Additional details of the technique of preparing Styrofoam models are discussed in THIS JOURNAL, 30, 503 (1953). Nor.ma. C. R.. J . CIIEM. EDUC.. . 27,. 504 (1950): ihid.,. 32,. 23 (1955). NOLLER,C. R., "Chemistry of Organic Compounds," KT.B. Saunders Co., Philadelphia, 1951. FOWLES,G. W. A,, J. CHEM.EDUC., 32, 260 (1955).
VOLUME 34, NO. 5, MAY, 1957
group of A 0 and MO models were made and the constructiona,l details are presented here in brief form. It, is hoped that this information together with the accompanying photographs will help others in developing this very useful tool. The basic shapes from which the Styrofoam models were made included 3-in. eggs (2.4-in. minor diamet,er, 3.1-in. major diameter), 2.5-in. rod, various sized balls, and plank." All of the wire frames were brazed from 3/32-in. melding rod. Any necessary carving or reshaping was done with a coarse wood rasp follomd by sanding with emory paper or with rough Styrofoam. For gluing, commercially available polyvinyl acetate adhesive was found to be a superior material. Color coding was thought to be one of the most important considerations in the model design. It is futile to build models whose meaning or symbolism is not quickly understood by the student. Accordingly, 5 All are available from the Star Band Company, Broad and Commerce Streets, Portmouth, Virginia, or l o r d P.orist supply houses.
we chose colors whose first letter was the same as the designation of an AO or MO. Scarlet was selected for s and sigma orbitals as well as the background color for hybrid sp, spZ,and sp3 orbitals. All p and a orbitals were dyed purple. Hybrid orbita!~were represented by a single purple dot on a scarlet background for sp, t,wo purple dots on a scarlet background for sp2,and three purple dots on scarlet for sp3. The hybrid symbols were made by first printing one or more black circles on the scarlet background with fountainand Acetylene
Boltom row, lcfl to i i g h l (a) s p ITybrid orbitals, scarlet with one puwle dot. A modol of s.sinplc s p hybrid orbital would be uaeful t o emphasire the point that two eleotmns can be present in eaoh of these orbitals. seemindy e a ~ s b l oof containing only one election. (6) s p Orbitals witb 2p eleotrons and 2 white eggs t o indicate imfilled orbitals as in oarhon. (a) sl;' Hybrid olbit&ls,scarlet with two omp pie doer. The mall sphere of eaoh orbital has been omitted bnt can be placed on the protruding wires. (d) spa Orbitals with one p electron and one vacant orbital indicated b y a white egg at the rear. Two of these ao' orbitals oeshed tonether w m l d result in the diradical ethylene molecule diaphyed st tl>eleftof the top row. ( a ) apr Hybrid orbitalr, ecerlet with zhree pirrj~lledots. The emall spheres representing the minor lobe of eaoh orbital lhave been omitted. TOProw, left to right: (o) Ethylone molecule shmving p electron on each carbon with a vacant p orbital adjacent, i . e . ethylene as a diradieal. Tbir picture of ethylcne ia taken at a sliphr anqle, as tlmugh the M O model were being viewed along the hxir of one of the C-H bonds. Tire hydrogens h a r e been omitted for simnlieity, b u t the s n s orbitals of the carbon ready for bond formation with the hydromns arc indicated b y the aesrlet eggs with t m purple dots. The C-C sicma bond is represented by a scarlet ball with t m "urplc dots m d a line underneath. The l, orbitals are. of course, above and below tho plane of the C-EI knd the C-C bonds. (b) Ethylene moleoule siter formhtion of bonds. I s in the preceding model, the hydrogens h a r e been omitted for simplicity. (c) Acetylene molecule ae h tefrsradioal, i.r. with four p eleotronr, two on each oarbon immediately adjacent t o t r o unfilled orbitals (white e m r l on the other carbon. The hydrogens hie omitted b u t the s p orbital with whirh the hydrogen would form a bond is shown by the ~isnhlsoarlet ball ~ i t ah ainglo purple dot. ( d ) doetylene moleoulr shorvin%formation of bonds between adihoent p orbit&. (el Acetylene, shoi,.. ing the spherical aymmetry of bonding in the molecule. ~
Figure 1.
Hydrogen a n d
.q
Atomic Orbitals
Bollom row, lrft lo iiohl: (o) Hydrogen atom i n tlm conventions1 molecular model on a mnlc of 1 in. = 1 A. (b) Hydrogen stom on the scale used for the A 0 and MO model. (2 in. = 1 A,). (r;) oiagrsmmatio oioss section of the iwdrogen stom to indicate that the position of the largest frection of the charge is in the volume element near the "Bohr radius." (A true cross seotion would show elmetro" charge density "uitelarge a t the center, decreasing outward to the ran der Whhk TD~,CTC.) . (dl .~ An alternate method of reoresentation of the position of the lsrgest fraotion of the eharce with h small sphere tbc rise of the Bohr radias. (a) Hydro~enmoleonl~on .e 1 in. = 1 A. eeale. (jI Cross section of a hydioaen molecule on t h e usual scale of 2 in. = 1 A. As in the cross section of the hvdroeen atom. this is diszrammatio in . t h a t the shading represents tho volume elements of the largest fraction of t h e electron charge. (The charm deneity is not zero near the nucleus, b n t this repleaentation was thollcht to be most uneful far the prosentstion t o elementary students.) Top row, Icfl lo iioht: (a) la Orbital, half resrlet t o indicate thht only one electron la prcrent, nlthough the wave foim is spherical and the orbital can aooammodate two electrons. Two inches in diameter. (b) 1s' Ovbital, 1.8 in. in diameter. (c) Alternate method oi representing the la'orbitsl: half sosrlet and half atrined to indicate ooooaite siens for the wave functions. .. (Sine should be the same as t h e other 1 x 3 model. We have not used this rrl w ~ t . n i n l # ~I.C..III-P ~#t II.c..~III.I~I. r 8.1 8 1 ~ I ~ ~ ~ ~ ~ . +1 s8. r )t #, 1 (fd. % ~ 28U:I>>ml, : 7 I . i I . . . ' c ?s( O:h~ta,. ( I C:
