R. T. Sanderson State University of lowo Iowa City
Valence:
A Laboratory Exercise
for General Chemistry
Teachers may he interested in a new laboratory exercise on valence recently tested by 600 general chemistry students a t Iowa. In this exercise, each student carefully examined each of a set of thirteen different atomic models, writing down certain pertinent observations and independently-reasoned conclusions about them. The models are all Styrofoam spheres of like size (2'/2" d)' on which the outermost orbitals (s, or s and three D) are reuresellted bv enameled-headed unbolsterv nails, moistened with glue, and pressed into the plasttc until the heads are flush with the sphere surface. White nail heads represent electrons, and black heads represent vacancies capable of accommodating electrons. A pair of white nail heads indicates a filled orbital and a pair of blaok nail heads, an empty orbital. A pair of one white and one black nail head represents a half-filled orbital-in other words, an unpaired electron. (Alternatively, the electrons and vacancies could be painted on the Styrofoam surface, either as black and white dots, or by some other designation such as minus signs representing electrons and zeros representing vacancies.) The empty orbitals are lo-
';P .. I wted t : l i t ~ : i 011 l the sphere S I I T ~ ~ L C Ilut ~. orbitals with elfx:t~onsare located according to their known directional properties: two such orbitals, a t opposite sides of the sphere; three, at the corners of an equilateral triangle around the sphere such that bond angles would be 120"; four, at the corners of a regular tetrahedron, angles 109"28'. A convenient way of locating three of the four comers of a tetrahedron on the surface of a sphere is to cut a circular hole in stiff cardboard, of diameter 0.94 times the sphere diameter. Mark a t the edge of this hole the corners of an
equilateral triangle circumscribed by the circular hole. Hold the card with the sphere fit snugly in the bole, and mark these same triangle corners on the sphere. These are three of the comers of the tetrahedron; the fourth can he located visually as exactly opposite the triangle. For atoms with two, three, or four outer shell electrons (such as Be, B, and C), two models are provided, one representing the ground state, the other the valence state. For example, one model that might represent carbon sho~vsan electron distribution of s2, p,, p,, and the other, s, p,, p,, p,. The thirteen models in the set have the outer electronic structures of H, He, Li, Be (2), B (2), C (2), N, 0, F, and Ne. All the inert-shell and 18-shell elements are thus represented. Each model is labelled with a coded letter. A typical set of 13, for which the material cost was about one dollar, is shown in the Figure. One set sufficesfor each group of ten students. This exercise was assigned only after completion of a series of classroom lectures to the students on electronic configurations and chemical combination. They were further prepared by a written description of the models and review of the principles of valence, issued to them several days in a d v a n ~ e . They ~ were asked, after they had read this information carefully, to examine each model individually and record on a chart the following data: (1) code letter, (2) whether ground state or valence or both, (3) number of covalent bonds possible, (4) approximate bond angles expected, (5) number of coordinate covalent bonds possible, (6) whether electron donor or acceptor, (7) most probable formulas of its binary compounds (if any) with each of the other models (making no exceptions for possible intermetallic compounds where ordinary covalent stoichiometry is usually not observed), (8) underlining of the code letter of that element expected to become a t least partially negative in the compound, and (9) a list of all actual elements (except transition elements) which each model might represent. T i e required for the completion of this exercise was about two houn. It gave students an opportunity to visualize and think directly about the electronic nquirements for, and results of, chemical bond formation. The exercise also gave them a better appreciation of the significance of the electronic similarities underlying the classfication of the chemical elements into groups of the periodic table. Reactions of both students and instructors were highly favorable. Support of this work was provided in part by the National Science Foundation, Course Content Improvement Section, and is gratefully acknowledged.
Obtainable in white, red, blue, and green from Star Band Co., Portsmouth, Virginia.
a Teachers vishing s copy of the full information and instruetians for this exercise may obtain it by writing to the author.
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Volume 37, ~ u m b e r 5 Moy , 1960
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261
