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In 1941 the Dutch chemist, Anton Eduard van Arkel, proposed an attractive triangular diagram (Fig. 1) to represent the progressive transition between the three limiting-cases of ionic, covalent, and metallic bonding (1). Given its pedagogical appeal, it is rather surprising that so few textbook authors have made use of this representation (2-7), perhaps because chemistry texts seldom provide a substantive discussion of intermetallic compounds and alloys.
As can be seen from an examination of Figure 1, van Arkel's original diagram had no quantitative coordinates. He merely guessed the location of each compound based on an intuitive estimate of its relative ionic and metallic character. In addition, he showed examples of progressive changes only on the outer edges of the diagram, thus leaving open the question of whether he viewed the diagram merely as three line segments with their ends joined or as a true solid triangle with compounds of intermediate character located within the triangle as well as along its edges. Later users of the diagram have adopted both points of view. Some have continued to show only edge transformations (4, 5); whereas, others (3, 6-7) have followed the lead of van Arkel's colleague, the Dutch chemist, Jan Arnold Albert Ketelaar, who in his 1947 version of the diagram (Fig. 2) implicitly placed compounds within the body of the triangle on a series of horizontal lines, though again the exact criteria for these qualitative placements were not given (2).
More recently, Allen (Fig. 3) has attempted to rationalize explicitly the horizontal lines in Ketelaar's version of the original diagram (8).1 Though Allen's description of his placement procedure is confined to a short and largely uninformative figure caption, he appears to have constructed his triangle, using only the elements in row three of the periodic table, by placing the elements in question at equal intervals along the base of the triangle in order of increasing configuration energy (which is Allen's euphemism for electronegativity). He then divides the triangle into equally spaced horizontals. On the first horizontal he places at equal intervals the binary compounds corresponding to each pair of elements on the base-centering it between the symbols for these elements. Thus, MgAl appears on the first horizontal half way between Mg and A1 on the base;...