A Simple Inexpensive ~ o d efor l Student Discovery of VSEPR When reaching the VSEPR theory tr, high schud and c d e g t preparatory d t t & u t \ I haw encwntrred reactions from in u,hnt you call a studenw such nr "We'll have to take your word for it that fuur rlactrou groups will arrange th~.mwl\~es wtmhcdml structure", wen when they were precenwd witha flexible plactlr trtrhnrdrsl ~nodcl.'rhereft,rr,osnn introduetwn to the chapter on molecular geometry I have employed the use of a simple inexpensive model with which the student discovers by himself the sense,behind molecular shapes. The model consists of two 60-cm long lengths of orange yarn with their centers tied together, thereby forming four ends. On each of the four ends are tied wooden halls, four centimeters in diameter, with a hole drilled through the diameter in order t o facilitate easv tvine. . ~. Models were prepnwd tiw r w r y groupof three s t u d ~ n t in s the dais. Each student triplet uas requested toset t w d the I,allim faraparr n$possihlr idisreynrdinl: theuther two hallst as i f the imll,and the 1engthsuf)arnwere rrp~llingcWh uther wry strongly. Wlrh the a d ofn purractor thestudenti recurded rhc angle between the twu lrnythsuivarn \Virhuur any d i f f ~ u l t all y uf the rrudcnt gruups achieved the linear rodiyumtion. The inmr procrdurr was peritmmi tor three imlls. thii time with the itudtnts alro rrmrding the dismnrr* hetu,cen each pmr uf l,alls, eltht-r uirh a tape measure or n shoelare. .\gain, without diiiirults all 01 the groups fwnd angles of 120" nnd thnt the dt.rnnrei b e t ~ e e nthe pair< uf balk u c r e all r w n l . M h m asked r u nrrnnre all fimr halls in the "f:,rthert mart" iuatisl srranrrment the intuition ut tnu-third.*of thc &ass led them to the tetrahidral structure, recording the angles as "about 110'%and equal distances between the pairs. But, one third of the class recorded equal angles of 90°, obtaininga planar X-shaped form. Their recorded distance between the pairs were equal because they selectively recorded distances hetween adjacent balls. A debate between the groups of students fallowed and was resolved when one of the "tetrahedral-structure students" demonstrated that by rotating one of the adjacent pairs of balls of the X-structure by 90' the distances between the pairs became greater and all are equal. The results were recorded on a chart on the blackboard including the number of repelling groups involved, the angles between the lengths of yarn, and the distance between the pairs, and I added the appropriate geometric term (linear, planar, tetrahedral). It is reasonable that this method could also be used for student discovery throughmanipulation of the trigonal hipyramid and the octahedron using five and six balls respectively (one added length of yarn), increasing the group to four students (six hands olus a measurer). ~~
~
~~
~~~~~~
trends of changes that must be allowed when certain repulsion factors are stronger than others (e.g., lone pair-lone pair versus lone pair-bonding pair repulsion). Most important though, is that my students won't have to take my word far it any more. Marc Halpern Hebrew University of Jerusalem .Jerusalem, Israel
Volume 56, Number 8, August 1979 1 531
