Copper Is Yellow Ariel H. Guerrem' and H a w J. Fasoli Universidad de Buenos Aires J& L. Costa and Susana V. de Piccardo Universidad del Salvador Our students give dubious answers when asked about the color of metallic copper. However, they are no more confused than classic textbooks of inorganic chemistry (1-4),most of which classify copper as red. However, some observations suggest that copper is yellow: (1) the color of alloys, obtained by dilution in a reducing atmosphere of different metals such as tin and alurninum(~hosnhorbronze): .. (2) . . in a remarkable microphotograph'of cbpper(1) oxide matrix (5) usina 300:l maenification..it is nossihle to distineuish clearlv . yellow needlesif copper. In addition, anvone can oerceive that conoer is vellow bv means of a simple experiment published many years ago (9) and herein modified in its setuu. Take a short strin of conper, about 2 X 4.5 cm, and h a n 6 t from a glass rod axoss the top of a crystallizing dish approximately 10 cm in diameter and 5 cm high, which contains a few milliliters of methanol or propanone forming a layer nearly 3 mm high. The copper is lifted from the dish and is preheated to redness by holding it in the flame of anearhy burner, then it is hungimmediateIv across the to^ of the beaker. With some practice, the metal will maintain its temperature, catalvzina - - the oxidation of methanol to methanal or of propanone (enolic form) to methanal and acetic acid. Heated in the reducing atmosphere of organic vapors from the shallow vessel with this setup, the small copper strip shines with a bright yellow color, an "exocharmic reaction'' (7). This yellow color is completely different from the reddish
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400
Journal of Chemical Education
one due to heating the metal. The experiment is quite safe with reasonable precautions. In order to explain the yellow color of the group 11(IB) metals comer and aold, the delocalization in the two close sublevels - 1)d10-n~' (n - 1) d9 ns2must he considered. Silver is theexce~tion:its atomic radius (0.144 nm) is similar to that of gold, A d its first ionization energy is lower than expected by averaging copper and gold values. Thus, we can imagine that delocalization is inhibited in silver by a relatively high energy difference between the two external electronic sublevels (n - l ) d and ns, stabilizing the dl0s1configuration and justifying the colorless shine of silver. In this context we mav ouote a remarkable observation (8):"Silver is a white lustr&s metal, which appears yellow if th'e light he reflected from its surface manv times before it reaches the
in
-
-..The -. reddish color of copper is produced by CuzO, which
forms easily in air and a further dark hue comes from black CuO. Briefly, metallic copper is really yellow, like gold. Literature Cited
W.J.; G r i w l d , E. Inorganic Chemistry; Heath: Baaton, 1966.Spanish version: Ed. Hmerte: bare el on^ 1977.
1. Kleinbprg, J.: Argensiwr,
2. Moeller, T. Iwlgonic Chemistry; Wiley: New York, 19%. 3. Cotton, F. A,: Wilkinson, G. Aduonced Inorganic Chemistry; Inteteiena:
New York,
1966. 4. Pauling, L. The Natue of the Chsmieol Bond: Cornell University: New Ymk, 1960. Spanish version: Ed.Kapeluaz: Bueno. Aims, 1961. WeatGe~y,1986 5. ZeisaAxiophot:Photomicr~cop~I~rR~~~~~~hondRoutine;Zeiaa: Ihmll~tl 6. MeCu1loeh.L.J. Chem.Educ. 1946,23,524. 7. Ramette,R. W.J.Chem. Educ. 1960.57.68. 8. Mellor, J.W.Mellor'~Ma(ernInorgonie Chemistry. Wiley: New York. 1967:Chap. 26. See. 17.
