Rex E. Shepherd University of Pinsburgh Plnsburgh. Pennsylvanla 15260
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Lecture Projectable Atomic Orbital Cross -Sections and Bonding Interactions
Anyone who has lectured in an introductory chemistry core program has experienced the difficulty which students frequently have in forming a mental picture of how the most simple covalent chemical bonds are made. There is also a scarcity of convenient lecture aids which can he viewed with equal erne by all of the students in a large lecture room of the size found in larger universities.' Confusion for the student can be increased when hard sphere models are used to depict simple molecules such as Hz, CHI, HzCO, HzS04, Nz, and so on. The hall and stick models of bonds between small atoms have nearly universal use as lecture aids. We have found the hall models serve an essential, complementary role to the screen projectahle atomic orbitals of this report for the development of a student's understanding of molecular structure. However, for the novice student the hard sphere models provide no information ahout the compressibility and flexibility of chemical honds. The hard sphere models allow a student to locate nuclei in molecules, hut for students who do not readily think in t e r n of Coulomb's Law, there is a sudden loss of information by looking a t hard spheres! By his own experience a student knows that billiard halls bounce and roll off other billiard halls. He sees no reason whv hard soheres should stay in a pyramid a s in CHI. What is &issing in these models is the basic information that chemical honds involve electrons and that the electronic interaction occurs a t a distance from the loci of the hard ~ p h e r e s . ~ During lectures in an introductory course for non-science maiors I have used an imnroved bonding model to simulate the-overlap of atomic orhihs. These mod& may he prepared in a few minutes by the use of small styrofoam halls for the nuclei, copper wire, and the commercial toy spring commonly Slinkys are wired end to end in order to called the "~linky."~ produce a n s orbital cross-section. By drawing out a slinky loop which has been wired end to end as in the s orbital case and grasping the system a t the center, a figure eight crosssection of the p orhital type is obtained. These can be wired to maintain a permanentc&figuration. Wiring the slinky loop in an unequal fashion approximates the hybrid of an sp3 orbital cross-section. t w o p type units make a d orhital crosssection. A numher of these units are prepared using various diameter slinkys. When the cross-section is placed on an overhead visual aid projection system, a pattern of the s, p, d, or hybrid orhital is clearly seen by the entire class. These orhital crosssections have received verv favorable comments from undergraduate students, teackng assistants, and other faculty members. It is the maioritv . - oninion . that these models of atomic orbitals exhihit many useful features. 1) They properly exhibit the Heisenberg Uncertainty Principle by
their diffuse appearance about the nuclei. 2) They emphasize that an orbital is a volume element of defined
shape centered on a heavy nucleus. The nuclei appear as opaque circles on the overhead projection. 3) Different sized slinkys may be used to represent various n values; for any given n level there is an average distance that electrons keep from the nuclei within a given orbital. The relationship of the average radius to then value is readily seen. 4) Within then level the electronic density decreases with radial extension. This is detected as a less intense shadow pattern. 5) Orbital clouds are deformable by other electronic repulsions and nuclear attraction within small physical limits. The models are subject to minor distortionsto represent this effect.
Mcdel showing p orbna~overlap. 6) The orbital lobes of two or more atomic orbital cross-sections
readily overlap to reveal an increased electron density at the intersection of the cross-sections. In this way they project a simple pattern for the formation of a chemical bond from separate nuclei and illustrate the advantage that results for the particioatina . . nuclei. 7, They are useful in illurtrstion of theorigin of Morsecurve pw renrial surfares fur amall nuclear displacement. from the equiLarge draplacements are re5irtcd hy thp librium bond di~tanve.~ spring system. 8) The quivering motion of the spring system imparts a psychological effect for the audience of an enereeticallvactive svstem capable of a chemical dynamic process.' Coupled with the ingenuity of the lecturer, a number of the features of chemical honds can he auicklv illustrated. For example o bonds formed by overlap of twos orbitali ( a n he used to illustrate linear diatomic molecules.. ex. H,. -.1.i.). etc.. or even the more complicated Hgz2f ion. Overlap oFan atomic orhital with a D atomic orhital results in bonds such ~ H a . Use of the sp3 hybrid orhital as those found in H C or and an s orhital will clearly show that overlap is easier and more complete for the larger lobe than with the small lobe. A triumph of these models is shown in the figure. Students have often expressed to me and my teaching assistants an element of disbelief that a p orhital of one atom can overlap a p orhital of a neighboring atom to make a s bond. The usual hand-drawn pictures on a blackboard or overhead svstem fail to convince students that atoms can get close enoughfor p orbitals to overlap a t all. These p orhital cross-sections rapidly eliminate this view because the slinky cross-sections readily
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The projectahle orbital cross-sectionswere tested on a class size of 300 students. The projectable models are intended predominantly as an aid in the valence bond description of chemical bonds which finds widespread use in introductory chemistry programs. Slinky is a product of James Industries, Hollidayshurg, Pa. 16648.
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Cornplemencary aid is any i,f several film packages such as the Chem Studies film "Chemical Bonding." Volume 55, Number 5, May 1978 / 317
interpenetrate to rreate the model of a a bond. During the survey of the periodic table portion of our course we attempt to show that the strength o f a rr bond between a second row element with a first row element is Doorer hv virtue of' smnllrr overlap than the corresponding p - p overlap fur elements of the same princi~alauanrum number. This feature is readily shown by the projed
