The Alums: Interchangeable Elements Msgr. Luckey High school ( in a

---

Jack 1. Lambed Kansas State University Manhattan. 66502 and Michael W. Lambert Msgr. Luckey High Manhattan, Kansas 66502

school

1 The Alums: Interchangeable Elements

(

I

in a Versatile crystal structure

T h e authors became interested in the crystal structure of alums during the course of a discussion about possible science projects. The subject was not used as a project, but the authors did satisfy their curiosity about the crystal stmcture. Alums are favorite compounds for growing of large, well-formed crystals. Detailed recipes are available for growing alum crystals, including '%nixed crystals" of two alums to demonstrate isomorphism.' Alums crystallize mainly in octahedra, but the addition of foreign substances such as urea or borax to the solution will remess the octahedral faces so that other shapes are obtainkd. The empirical formula for the alums is M(I)Mf(III) (S04)2.12H20. The monovalent cation, M(I), may be any of the alkali metals except lithium, plus thallium, ammonium, hydroxylammonium, methylammonium, or hydrazinium. The trivalent cation, M1(III), may be aluminum, gallium, indium, titanium, vanadium, chromium, manganese, iron, cobalt, rhodium, or iridium. The sulfate anion may be replaced by selenate or partially replaced by t,etrafluoroberyllate or tetrachlorozincate. Water is the only unchanging component, and even the hydrogen in the water can be replaced by deuterium. Few other classes of compounds exhibit so wide a variety of interchangeable elements. The crystal structure of the alums is basically that shown in expanded form in the figure. The model is HOLDEN, A,, AND S I N ~ RP.,, "Cryst& and Crystal Growing," Anchor Books, Doubleday & Company, Inc., Garden City, New York, 1960, pp. 109-111. WYCKOPF, It. W. G., "Crystal Structures," (2nd ed.), Interscience (division of John Wiley & Sons), New York, 1965, pp. 872-8.

constructed of cork balls and thin bronze brazing rods. The atoms are painted in distinctive colors for easy identification. More nearly accurate representations of the water molecules and sulfate ion are achieved by paring segments from the cork balls before cementing to indicate interpenetration of atoms. The unit cell is depicted, but the water molecules and sulfate ion are shown for only one-eighth of the unit cell. The two types of cations are seen to be in a sodium chloride, face-centered cubic array. Six water molecules are coordinated about each cation, with greater crowding and contact around the smaller, more highly charged trivalent cation. W y ~ k o f fwho , ~ gives details of the positions and parameters of the atoms in a number of alums, describes the structure as a cesium chloride, body-centered cubic arrangement of hydrated cations and sulfate anions. This concept is valid only if all the hydrated cations are considered to be equivalent.

'

Unit collof the alum rtrwture.

Volume 47, Number 6, June 1970

/

465

Three subclasses of alums exist. Typical of the most common, or a,form is alum itself, potassium aluminum sulfate dodecahydrate. If the monovalent cation is cesium, its larger size permits close approach of oxygen atoms from six neighboringsulfate anions. The number of oxygen atoms surrounding the monovalent cation is

466 / Journal o f Chemical Educafion

thus increased from six to twelve, resulting in the 8alum structure. When the monovalent cation is small, the orientation of the sulfate ion in the crystal lattice is different and a third, or y, form results. Sodium aluminum sulfate dodecahydrate (sodium alum) is the only y-alum that has been studied in detail.