An Easily Constructed Trigonal Pyramid Model Shukichi Yamana Faculty of General Education, Kinki University, Kowakae, Higashi Osaka 577, Japan
A model of a trigonal pyramid having t h r e e t o p angles of about 90" that i s useful for teaching stereochemistry (especially t h a t of arsine)' can h e m a d e easily b y using a sealed e m p t y envelope. T h e steps are illustrated a n d given below. (1) The first figure shows the starting configuration, which is produced by steps 1-6 of a previously published a r t i ~ l e . ~
(2) The intersection of the line AD and (GI is marked I. (3) A horizontal line, perpendicular to the line IG) at I, is drawn so that it crosses the left- and right-hand sides of the envelope and the line IF) at J , L, and K, respectively. (4) The right lower part of the envelope is folded up at J s o that the line J L falls on the left-hand side of the envelope. The new corner on the right-hand side of the envelope is marked M. (5) The right lower part of the envelope is unfolded. (6) The intersection of the line J M a n d (G) is markedN. (7) A horizontal line, perpendicular to the line IG) at N, is drawn so that it crosses the left- and right-hand sides of the envelope and the line (Flat 0 , Q, and P, respectively. (8) On the hack of the envelope, the points corresponding to the N, P, and K, are marked N*, P a , and K*, respectively. (9) The upper part of the envelope is cut off along the line OQ. (10) The base of the envelope is cut along the line AB. The original points H and F , are rewritten as (HI and Hh) and IfI and F A resoectivelv..aceordine to the side on which they are situated If = front and b = hack).
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113) The right- and ldft-hand sides of the envelope are pushed together so that the three lines (JK, KK*, and K*J) form a regular triangle. 114) Three points Hi, B, and Ha, are pushed together, and the shaded creased triangles (HIKE, BK*Hh,and HrJXa) are folded inside. The sides are taped together to form the hottom of a trigonal pyramid model. 115) In order to make the next step smooth, the reverses of the three creased triangles are folded in and are pushed against the inside wall of the model mentioned above, by lying along the three sides of the interior bottom. (16) Three points N, Q, and N*, are pushed together and the remaining shaded creased triangles (NKQ, QK*N*, and NJN*) are folded inside. The sides are taped together to produce the model given in the title.
' Nielsen. H. H . J. Chem. Phys. 1952,20.759 Yamana. S. J. Chem. Educ. 1987,64,761.
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111) Oblique lines are drawn to shade six triangles (NKQ and KHIB on the front; N*K*Q and K * H a on the hack; and NJN* and JHfHa on both sides) I121 The cnveic,pe is iolded hmh hackwnrd and forward along the lines J L , . V H l ( i . r...V'H.J,PFi(i.e.,Pf.r, JN1i.e ...I ~ \ " J . K K ci.r.. hr"A",. KG tie.. K'OI. J H , ( i c . JH.1. H,K lie.. HX',.
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