ADAPTABLE ELECTRONIC MODELS SIDNEY J. FRENCH Colgate University, Hamilton, New York
M
Many of the models now in use which have played important and useful r8les in the past do not meet these simple criteria. The cubical framework so often used is both awkward and misleading. Attention is drawn to the framework. The pairing tendency of electrons is not illustrated nor are the forces acting between nucleus and planetary electrons demonstrated in any way. In showing a single valence bond, it is necessary to unite the cubes a t two corners, thus making i t awkward to interpret the electron pair as a single bond. When several frames are placed together to represent a molecule, i t is well-nigh impossible to discover the molecule because of the contortions of the frames. Models in which the nucleus is represented by a solid sphere or cube with electrons projecting from eight positions are somewhat more . =.t' .. s a t i s f a c t o r y . However, similar objections can be raised, for no pairing is shown and two wires or a+s m u s t be used to represent one bond. In attempting t o overcome these objections, t h e writer has developed a model which is simple and adaptable. A ATOMICMODELSOR THE ELEMENTS LITHIUMTO NEON pliable rubber The comparative sire of each atom is represented by use of the proper length of bond wires and proper placement of cross-arms. The helium pair of electrons is fas- ball with f o u r tened to the nuclear body with pins. Labels can be pinned to the balls if desired or openings intetrasymbols or formulas may be written on white-headed thumbtacks.
ODELS of one type or another have been used to illuminate the atomic and molecular theories since the days of Dalton. They represent the scaffolding supporting the abstract concept of the theory. Like the scaffolding they must be established iirst and when no longer needed as aids to the mental processes they must be removed. When properly used they are a most valuable aid in establishing generalizations. With the advent of the electronic theory of matter and with the ever-growing significance of atomic structure and periodic relationship becoming rapidly recog-. nized in newer elementary textbooks, the need for adaptable models, which will carry the student by easy stages from the simple atom and molecule to the electronic concept in all of ,- .. .. :.ww,. .-: ,.- '' its ramifications, becomes increas* ;\. .. '. .I inglv important.
..
satis is factor^
model should be simple enough so that the mind of the observer does not dwell unduly upon details nnrelated t o t h e theory. Itshould also permit an easy transition to the abstract concept and t o methods used in representing the concept in mon practice.
'
Perhaps the most noteworthy feature is the method of attaching the cross-arm to the valence bond in such a manner that it may be quickly put in place or removed
MOLECULAR MODELS01. METHANE(LEFT) TETRAHALIDE (RIGHT)
AND
CARBON
The positions of the bonding pairs of electrons indicate the relative electronegativity of carbon in methane and its electropositivity in carbon tetrahalide. The figure in the center shows the few parts necessary to a set of these models.
hedral positions is used for the nucleus or kernel. Spring wire bonds with booked ends may be frictionally engaged in the openings to serve as ordinary valence bonds. Ordinary valence relationships are shown by simply engaging the free hooked ends of the bonds together, giving a rigid union quickly released. Short and so that, while it is rigidly held, it can be moved cross-arm wires may be attached to the valence bonds along the bond a t will. This feature makes it possible by means of small rubber bands. The method of at- to represent any degree of polarity in a molecule by tachment holds the cross-arm rigidly in position, yet simply moving the bonding pair of electrons toward the permits it to be moved along the valence bond a t will. electronegative element. The advantages of such flexiSmall rubber objects can be placed on the ends of the bility in teaching polarity are evident. Because of cross-arms to represent electrons. When eight elec- this feature, it is also possible to represent the relative trons are in place around a nuclear body the general sizes of the atoms, and, as is well recognized, the size effect is that of a modified cubical arrangement, though, of the atom plays an important r61e in determining the of course, the tetrahedral arrangement is also retained. chemical properties of the element. With the older The notable difference is that the electrons are definitely type of model, it is difficult to represent either polarity shown as paired and each pair is associated with one or relative size without a series of different-sized frames, valence bond. thus reconciling the electronic structure for the electrons are ~enerallv ' shown in fixed oositions. With these models it is with our usual representation of the bonding pair therefore possible to start with the simplest model as representing one bond. consisting of ball atoms In representing atoms and valence arms and pass of elements, the ends of in turn to the electronic cross-arms not filled by structure of the atom, electrons represent t h e electronic structure of the negative valences of an molecule, polar, non-polar, element. Thus, carbon is and semi-polar valence, shown as having four elecpermanent and temporary trons, one on one end of dipoles, and coordination each of the four crossvalence. The transition arms. The four free ends from the niodel stage to represent t h e negative the abstract concept is valences. Electropositive simply and easily made elements are shown by since the models bear a placing the cross-arm in decided resemblance to the such a manner that only plane representations of the end bearing the elecelectronic symbols a n d tron protrudes (see figure, formulas. p. 564). One of the holes If students themselves in the ball extendsthrough, are permitted to work with thus making it possible to such models in the laborause opposite as well as MOLECULAR MODEL OF CHLOROBENZENE tory, they quickly develop tetrahedral positions. In The relative electropositivity of the carbon atom to which images which are of incalthis manner any configur- chlorine is attached is shown by the position of the bonding culable value later. ation can be shown. pair of electrons.
