R. J. FRIESEN, editor Unherrity of Woterioo Woterioo, Ontario, Conado
Keep Chemistry Simple! W. B. Pearson University of Waterloo Waterloo, Ontario N2L 3Gl
I read with interest the admonition, "Keep chemistry simple!" in CHEM 13 NEWS following an article which appeared in Inorganic Chemistry reporting the preparation of the compounds, ErBsa*r, DyBm, and ThBx. Actuallv. - . thines are seldom what they seem and these high horides a r e t h e most beautiful example of simplistic eeometrv. One of the most attractive geometric figures is " the icosahedron in which twelve points, or atoms, are arranged on the surface of a sphere so that each has five surface neighbors, and the atoms severally form twenty equilateral triangles over the surface of the sphere (Fig. 1). If an atom at the center just touches twelve equalsized atoms located a t the apices of an icosahedron, then the surface atoms are not themselves in 'contact'. The hectohoride is built up of roughly spherical groups of 156 B atoms that are packed together in two orientations in the same arrangement as the individual atoms in the rock salt (NaCI) structure (Fig. 2). These groups of 156 atoms or "~seudoatoms" that pack in the NaCl arrangement are themselves made up of twelve "pseudoatnms" each~ comoosed of 12 B atoms in an icosahedral ar~ ~ ~ rangement that pack together icosahedrally about a central "pseudoatom" of 12 more B atoms in icosahedral arrangement, giving 156 B atoms in all! In fact, the structure carries icosahedrality to its utter limit!-or indeed, "larger fleas have lesser fleas on their hacks to bite 'em!" ~
Figure 1 . The icosahedron whose apices are successively the locations of B atoms, and B q 2"pseudoatoms" in the ErB.6 structure.
packed in the rock salt arrangement, and this is filled with the remaining boron atoms and the rare earth atoms. The structure was determined for YBMby S. M . Richards and J. S. Kasper.
An Alternative to AgN03 Interaction of Metallic'Zinc with Aqueous Lead Acetate Helen L. Williams, Western Dubuque High School, Epworth, Iowa, sent us this experiment which was planned by J Schmidt, from the State of Washington, a t a National Science Foundation Summer Workshop in 1966. Problem To find a replacement for the copper-silver nitrate system in CHEM Study Experiment 7-Copper-Silver Nitrate Reaction. Limitations 1) Reaction must he substantially less expensive than the copper-silver nitrate system. 2) Reaction must give results as good as the copper-silver nitrate system. 3) Students must he able to complete the experiment within two 50-min periods. Introduction CHEM Study Experiment 7 uses copper to replace silver from a 0.24 M silver nitrate solution to provide an experimental determination of the mole ratios between silver and copper, and to furnish a basis for writing equations. Since silver nitrate is very expensive, i t is desirable to have an alternate, less expensive reaction available. After preliminary trials with zinc strips, zinc dust, iron powder, and magnesium ribbon with solutions of lead acetate and lead nitrate, the zinc metal-lead acetate solution was selected as most promising. The solution was then tested a t various concentrations. The concentration giving the most consistent results was 2.65 x M (2 g lead acetate in 200 ml distilled water). A few drops of glacial acetic acid was added to clear the solution. (The p H was about 6.) Procedure Follow CHEM Study Experiment 7, and add an additional step of washing the lead deposit with acetone. Conclusions 1) The zinc metal-lead acetate gives results as good as the copper-silver nitrate system. 2) The experiment can he satisfactorily completed in two 50-min class periods. 3) The reaction is much less expensive
Figure 2. The NaCi arrangement whose sites are the centers about which B,. pseudoatoms are located in two orientations.
Needless to say, there is considerable space between the "spherical" 156 atom "pseudoatoms" when they are
Lead acetate Silver nitrate Zinc, granular Copper foil
Pounds 5 5 1 1
Price ($) 12 236 4 5
Volume 52, Number 6,June 1975 / 391
