Nuggets glitter. Each of the close-bound atoms of any bit of the coinage metals pools an outer electron or two to form a sea of mobile electrons, the physical origin of metallic properties (see our December 2000 column). Here we contrast those rare and showy elements with two other metals, sodium and potassium, both more abundant and more important--yet seen in their pure state primarily in the chemical industry.

Today sodium is, like aluminum, an inexpensive metal. A fresh-cut silvery surface of sodium, soft as wax, covers itself at once with an ugly, rough rind, like some mineral cheese. Toss a small piece into water and watch it float as it dissolves, "dancing frenetically and developing hydrogen" (Primo Levi). Potassium, its heavier sister element in that family tree of elements we call the periodic table, could be described with the same words. But a pea-size dollop of potassium not only reacts vividly in water to release hydrogen but then ignites the hot outpouring gas!

Lustrous metals are not for the human organism. Modified atoms of sodium and potassium are both requisite for human life, but no tiny nugget will be found within your body. There they reside mostly in aqueous solution; a reacting atom of either one loses its outermost electron in water, to drift as a now charged atomic core, called an ion, amid the throng of water molecules, its positive charge balancing some far-wandering electron. Compounds such as sodium chloride (table salt) and its counterpart, potassium chloride, are familiar as stable, colorless crystals, precipitating, among many others, from evaporating saltwater. Typically relics of evaporation over geologic time, such natural deposits exist worldwide, wherever any body of water has been cut off from its sources.

This article was originally published with the title The Needy Porcupine.

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