A Mnemonic for Oxy-Anions For two decades I have been telling students of general chemistry that they just have to memorize that there are three oxygens in nitrate and chlorate, four in sulfate and phosphate, and so on and that they must also memorize the charges. Then it dawned on me that there is a mnemonic rule, shown on the periodic table in Figure 1. Period one and the halogens have three oxygens in their "ate"oxy-anions, while the black in the center have four. The charges are shown in Figure 2, and the easily remembered relationship is apparent.
Figure 1. Number of oxygen atoms in oxy-anions wilh the
Figure 2. Charge on oxy-anions.
I am not sure whether it is more confusing than helpful to point out that the halogens have three only because of the accident of history in which their XO; ions were called "per.. .ates". Had it been otherwise, the nomenclature would have been even more straightforward. For more able students it may he interesting to add that the unmarked elements in Groups 13-16 also have XO; ory-ions but that they are complicated by the prevalence of meta-acids and hydroxy-anions. This will require elaborate explanation and will confuse the less able. I t is helpful to point out that -ates, -ites, per. .ate%and hypo.. ites of the same element all have the same charge.
Steven J. Hawkes Oregon State University Corvallis. OR 97331