edited by
J. DUDLEY HERRON Purdue University West Lalayette. Indiana 47907
Inexpensive Space-Filling Molecular Models Useful For VSEPR and Symmetry Studies
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L. Chapman 769 Selkirk Street hncouuer, B.C. V6M 2Y5 Most chemistry teachers have difficulty in acquiring large, expensive molecular models for display or demonstration. is possible to construct laree models from scraps - of plvwood, .. srdhoard, or light plastic sheet a t small cost. The models described can be constructed on a scale to suit le teacher's needs. The set described was huilt on a scale of mghly 6 cm/A or a magnification factor of lo9. A radius of cm was used for the central atom, aradius of 5 cm was used a terminal atoms, and 7.5 cm was used for bond lengths in 1models. For even larger models, one could double or triple iese dimensions. onstructlon Details
The general method of construction consisted of cutting flat ieces from sheet plywood, hardboard, or plastic to replicate le shape of the main symmetry planes of the molecules mcerned. These planes were pieced together either a t 90' 120' as required. T o join two planes at 90°, a slot, slightly ider than the thickness of the material being used, was cut I each piece, and the two pieces joined, slot to slot. In the few
atterns for model components.
798 / Journal of Chemical Education
cases where the planes meet at 120°.. snecial . clips or braces are required to hold the pieces togetht:r. The models can be made rieid and uermnnent bv rrluinr the planes together or they can be left unglued so thattheymay be disassembled for convenient storage. If the model is easily disassembled, a set with fewer pieces can he used to build any of the typical VSEPR shapes, including lone pairs or unfilled orhitals, as in the case of BeFz or BF3. When making only the minimum number of model parts, it is best to paint the central atom (A) in all pieces one distinctive color and to paint all the terminal atoms (B) a different distinctive color. If two sets of orbitals of each type are included, they may be painted to differentiate filled and unfilled orbitals. The accompanying patterns in the figure indicate the shapes that are required. Each central atom is 6 cm in radius, and each terminal atom is 5 cm in radius with a bond length of 7.5 cm. The slots are cut so that their lengths are one-half the distance from the edge of the plane to the nucleus of the central atom in all hut the orbitals. Here the length of the slot is halfway from the center of the orbital to the edge of the plane. The width of each slot is slightly wider than the thickness of the material; e.g., when using 0.3-cm thick hardboard, the slots were cut 0.35 cm wide. The table lists the quantities of each part required for each model. Those models huilt on the tetrahedral bond angle have their planes a t 120' and require special clips to assemble the
Number 01 Pleces Requlred to Produce Varlwr Models Typical Molecule 1
Model AT& AB& ABzEI ABLo AB&
BeF2 BF3 PbF2 CF4
2 3
4 5 6 7 8 9 10 11 12 13 14 15 16
1 1 2
1 3 1
Minimum pieces
XeF,
required far all models TDtal pieces required for all models
Model Labels
21
8 1 1 4
1. Position of the atomic nuclei 2. Covalent bond length 3. Internuclear distance 4. Van der Waal radii 5. Covalent radii 6. Lewis dot structure I. VSEPR formula;e.g. AB3E2 8. Molecular shape 9. Typical molecular formula; e.g., C1F3 10. Bond angles 11. Orbital labels 12. Symmetry labels-(Schoenflies notation) 13. Lewis acid or base
1 2 -w2 1 1 1 2 2 3 1 3 1 6 4 3 1 1 1 1 1
2 1 3 4 5(7)2 1 1 0 7 9(7)1 1 2 2 1
Number of Orbital Pieces Required
Orbital
Model
P P P sp3 sp3 dsp3 dsp3 dsp3 ff sp3 ff sp3
AB&
AWo AB&1 A W I AB2E2 ABlE A%EI ABzE~ AB5h AB&
Minimum required Total required forall models
17 18 19 20 4 4
2
6 6
Practical Difficulties
2
The slotted models are easily assembled, providing the fit is neither too tight nor too loose. Paraffin wax or soap act as good lubricants for the parts that come in contact. If the piece? fit too tightly, sand or file the slots slightly and re-lubricate the parts. When the parts fit too loosely, build the slot up with paint or attach a narrow piece of card or masking tape in the slot. To assemble the ABzEo model, slide parts 1 and 2 together, slot-to-slot, followed by two pieces of #3, slot-to-slot. Using the table and the figure chwse the appropriate pieces and join them slot-to-slot to complete the set. In assembling the s p s type of non-slotted models, decide whether you wish to use the 120' metal tabs, the 120' wooden supports, the 120' metal supports, or a strong adhesive tape. For these three models, you will need a minimum of 12 metal tabs each and ~ r o b a b l vmore. You will need 12 nieces of tape of suitable lengths fo; the supports. The tabs can be attached with epoxv due, usine clothes oees to hold them until the glur sets.'lhe &hc or supl)orfs sho"ldbe atwcherl so that the ridgeof thesuprlort matches thestraicht edge of the oiece numbered, 8 , 9 or i0. Glue the pieces together, assembling them a piece a t a time. When using adhesive tape, bind the pieces along all valleys with the edges in contact for utmost rigidity. When masking tape is used, the model can he disassembled, and the tape discarded. As a rule, the tape is more effective for lighter weight material such as cardboard or light plastic than for plywood. An attempt was made to produce: (1) models of ethane, ethene, and ethyne to show the presence of sigma and pi bonds, and, (2) models of the same molecules to show delocalized molecular orbitals. While models were produced, they became too complicated to he practical, especially in the case of ethane where 20 plastic clips were required to hold together 16 pieces of plastic sheet. I t appears that the method of producing molecular models described in this article is best suited to molecules with one central atom.
1
3 6
2 1 1 2 2
3 3 1 2 6 4 21 10
-
The models mav be used without labelline or thev, mav,have some or all of the following labels incorporated into the set b'or disolav purooses, c o m ~ l e t elahellme is a useful teachins adjunct. felt pen with pe;manent ink appears to be the bes; tool for markina and letterina the models. The following labels are suggested:
3 3
NHs H00
AB4E2
papers or essays. Choose the clip appropriate for the thicknesl of your plywood, hardboard, cardboard or plastic stock. Usin8 epoxy glue, join 3-cm lengths of the plastic clip. For clips t, accommodate four-winged orbitals, combine four pieces o 3-cm lengths of the plastic clip.
Number of Piece
1 2 4 2 10 7
models if they are to be disassembled later for storage. If these same models are to be permanent, the parts are easilvjoined by means of any strong-bonding adhesive tape or by means of valley supports cut from triangular wood strips having a 120" angle. If orbitals are desired, you will need two typs: (a) those with three wings (planes a t 120") to attach to models AB3Eo, AB2El. AB3E1, ABzE2, etc., and (h) those with four wings (planes at 90°) to attach to such models as ABzEo, ABBE1, ABzE3, AB4E2,etc., in order to match the symmetry planes of the molecules. Note that r, orhitals reauire two lobes of equal size while hydrid s p z b r sp3 orhitais have one large positive and one small negative lobe. For the purpose . . of lone pairs in models, we ignore the small lohe. The orbitals can be held in position by means of slots at the base or with special clips. The slots " x " in the three winged orbitals should be produced by cutting a piece out of the hase times the thickness of the wing. For the of each wing four-winged orhitals, a slot "y" should be cut in the hase of oieces 18 and 19 that is f i times the thickness of the wine. he orbitals fit into place so that the wings bisect the angles between the olanes of atom "A" where the orbital or lone oair reside. ~ and orhitals can be made Special clips for the S D models up from three pieces of blastic pressure clamps, such as the "NEW E-Z slide" type used as the spine in binding terms
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a
+
+
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Volume 55, Number 12 December 1978 / 799
