appeared are iefl on the screen. This is One of four animation modes.
are closer to CI and I (as shown in Fig. 151, emphasizing the effect of eiectronegativity.
amount of exaggeration needed for my students to immediof Cl~ttractinj:electrons atelv see the rreater electronegativity . more strongh than I. There are up to four zones available as bonding orbitals. Using five or six zones for honding on the screen gave a confusing picture; therefore, the tetrahedral, trigonal hipyramidal, and octahedral molecules are depicted using straight lines for bonding pairs of electrons. Figure 17 shows a lone pair of electrons in one of the coordinating positions. For the molecules with more than one oxidation state, one or two are always of the type that can he animated. The molecular structures are not stored as such but are derived from the number of outermost electrons in each atom, possible oxidation states, and the difference in the electroneentivitv between the two elements. In other words the al-gorithms used to make decisions on molecular structures are understandable bv freshmen in their first month of class. We have had good success using a combination of this program and handing out the algorithms used in the program and letting students use this step-by-step procedure to find VSEPR structures. Students have found it clearer and more meaningful to see and use an animated simulation of molecular bondine rather than just studying textbook descriptions and equatiork They get a feel for electrons moving, the changing distribution of honding electrons, and changing molecular shapes using the same atoms in different oxidation states. The level of interest in and understanding of bonding has increased. The program the door has oroved to he a useful tool which freauentlv. ouens . to more questions than it answers for users including the author. The program is written in UCSD Pascal with graphic extensions. The oromam . .. will run on a Terak, hut not on an Apple 11 because of the grdphirs. The program is large-over 100 blocks 011 an %in. diske~te.It ronsists of the main program, three compiled units in the system library, element data base, periodic table, fob, a utility program to allow additions, deletions, and corrections t o the data base. Package includes program and user directions and an 8-in. diskette. Send a check for $25.00 payable to Brian J. Pankuch to the ahove address. ~~~~~~
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available for Te with five pairs of electrons, four bonding pairs, and one lone pair.
tutions switching from the P E T to the Commodore 64 face the prohlem, however, of what t o do with existing programs written for the PET. Although some P E T programs will run on the Commodore 64 without modification, programs that involve peeks and pokes must be modified to accommodate differences in the memory maps of the two computers. An alternative is to utilize the "PET Emulator" program, which reconfigures the memory of the Commodore 64 to mimic that of the PET. One potential difficulty in using the Pet Emulator, however, is that it destroys the current program in memory every time it is switched from the P E T mode to the 64 mode (or vice versa). Although this is not a problem for a user who is manually loading his or her own programs, i t does create an obstacle if one has a computer network in which programs are automatically loaded under software control. Suppose, for example, that users were initially presented with a menu program containing a mixture of P E T and 64 programs. If the emulator is in the 64 mode and the user picks a PET program, the emulator must he switched to the P E T mode. Although this switchine can he trieeered from within the menu oromam. . .. the menu program itself will he lost as soon as the switch to the PET mode occurs. Hence the command to load the P E T program, which was part of the menu program, will be lost as well. In order to circumvent this problem, I have written a machine lanruare subroutine that carries out the loadinr! instruction f o r t h e next program after the basic progr&has been lost. This machine language suhroutine is invoked by putting an appropriate SYS command into the keyboard buffer before the P E T emulator is reconfigured. For example, to load a P E T program the following ASCI codes are placed into the keyhoard buffer (locations 631-638):
Location 198 is then poked with the number 8 to indicate that there are 8 characters waiting in the huffer. After the P E T Emulator is switched and the menu program is wiped out, the computer returns to its "ready" configuration. I t finds the ahove 8 characters in the keyboard huffer, which read: sY40715(carriage return). In other words, the computer now encounters the command SYS40715. At memory location 40715, I have put a machine language suhroutine that executes A Machine Language Subroutine for Automatic the loading of the next program. (The actual name of the Intermixingof program being loaded is poked into the appropriate locations Commodore 64 and PET Programs in this routine by the menu program prior to switching the P E T emulator). Lewis J. KielnsmHh Using the above approach, it is relatively easy to rreate a Depanrnent of B~ologrcalSclences Unlvers hr of ~ , ~ h mixed , ~ mmu ~ ~ot PI?T and Commodore 64 programs that automatically load one another. Copies of the appropriate rou~n~ Ardor,MI 48i09 tines, along with instructions for their use, are available from Project SERAPHIM on Commodore 64 Disk #l. Send a The recently introduced Commodore 64 microcomputer check for $4 made out to Project SERAPHIM, Acct. 20352 to: represents a significant advance over the earlier model P E T Department of Chemistry, Eastern Michigan University, cornputen in terms of memory, high resolution graphics, sprite Ypsilanti, MI 48197. animation, sound capabilities, and price. Educational insti792
Journal of Chemical Education
