Hands-on Models for Close Packing
One problrm in discwring dose packing structures is that students in our classes cannot see the models clearly. They may understand the imponance of tetrahedral and octnhedrnl holes [see Ho and Duuglas(1,l for deriving many ionic compounds, but they don't easdy see the holes, even when models are passed around. 1Ary don't eas~lygam hands-on experience without some effort. Ball bearings (or marbles or foam spheres) and wooden holders from 2- x 4-in. boards easily overcome this problem. They provide the opportunity for students to form the close-packed structures and to see the tetrahedral holes and octahedral holes first hand during the lecture hour of advanced inorganic chemistry. The materials may he easily amuired. Our storeroom had received a set ofaluminum hall hearinge (diameter 0.367 in., 9.3 nun) a s a @from a local firm.Cheao fir (2- x 4-in.) was cut into souares. and drilled in the center with a spade bit (1 3/s-in.) toa depth a little d- e- -m re -r ~than the soheres k e d . (Theedees bfthe wood holder were rounded off in a ;haper to eliminate shamedges. . .. but nou ugh holders were made so that two studrnrs coulh share a holder. thi* was n pcraond I h m , d u n g advanced innrgnnie rhemlstry class, rhr holders, conventently rarned in a shopping bag, were quickly passed out, and so wrre the hall bearrngs. Each unit needs ten; they ran be counted our quickly, and then the student
.
~~~~~~
v-uuu"
~~~~
~
.~
~
~
" .-...-
..-.
Several potnts uftnterest emerged. The mechamw (passing and collecting, didn't take much time. Students wcrc nhle ur see tetrahedral and octahedral holes and were ahle toree the dflerences in sires that wr described. Some commented that this was the first time that had really understood the concepts associated with close packing structures. a ball bearing. But I11 do better next time. What was really interesting: only one person dmpped ~
'HO, S.-H.; Douglas, 6.E. J. Chem. Educ., 1969,46,2007.
Dean F. Martin University of South Florida Tampa, FL 33620
Volume 69 Number 6 June 1992
495
