Polonium (Po), atomic number 84, occupies an ambiguous position on the periodic table. It was first isolated in 1898 by Marie and Pierre Curie, who named it after Marie’s native country, Poland. Polonium was the first element discovered based on its radioactivity, a feature that still defines its classification. The question of whether Polonium is a metal, nonmetal, or metalloid has long been debated by chemists. Its complex chemical and physical behaviors make simple categorization impossible.
The Borderline Classification
Polonium is located in Group 16, the Oxygen group, placing it directly on the dividing line between metals and nonmetals. This position is the main source of its ambiguous identity, as elements here often display properties of more than one category. Many sources classify Polonium as a metalloid, acknowledging its position on the “metalloid staircase” and its blended characteristics. Metalloids, like silicon or arsenic, have properties intermediate between those of metals and nonmetals.
However, Polonium is often classified as a post-transition metal, sometimes called a “poor metal.” This designation stems from its noticeable metallic luster and relatively high density, similar to neighbors like lead and bismuth. The debate highlights the fluidity of elemental classification, which is often based on consensus rather than rigid definitions. Polonium’s behavior is best understood by recognizing its dominant metallic tendencies, while still retaining some chemical traits of its nonmetallic family members in Group 16.
Physical and Chemical Properties
Polonium’s internal structure and behavior strongly support its metallic designation, particularly regarding its electrical properties. Unlike most metalloids, which behave as semiconductors, Polonium exhibits electrical conductivity that decreases as its temperature rises. This temperature-dependent conductivity is characteristic behavior of true metals. Polonium also forms two metallic allotropes at room temperature, including the simple cubic crystal structure, a unique arrangement among the elements.
Despite these metallic traits, Polonium retains significant volatility, a property more commonly associated with nonmetals. For instance, half of a Polonium sample can vaporize in air at only 55°C over 45 hours, which is low for a metal. Chemically, it tends to form compounds in the +2 and +4 oxidation states, resembling its lighter chalcogen relatives, tellurium and selenium. This mixed chemical behavior demonstrates the element’s difficulty in conforming to a single category.
The most defining characteristic of Polonium is its intense radioactivity, specifically the common isotope Polonium-210, which has a short half-life of 138 days. This vigorous decay profoundly influences its physical state, causing significant radioactive self-heating and generating about 140 watts of thermal power per gram. The heat is so substantial that a small sample of Polonium-210 can reach temperatures exceeding 500°C, emitting a faint blue glow due to the ionization of the surrounding air.
Uses and Extreme Toxicity
Polonium’s extreme radioactive properties, while hazardous, provide specialized practical applications. Its remarkable self-heating capability makes it a highly efficient, lightweight heat source for generating thermoelectric power. This utility has been exploited in space technology, where Polonium-210 was used to keep instruments warm in Soviet-era lunar rovers during cold lunar nights.
Polonium is also utilized commercially in anti-static devices and brushes due to its emission of alpha particles. The alpha radiation ionizes the air, neutralizing static electrical charges that build up in industrial processes like paper rolling or plastic manufacturing. When alloyed with beryllium, Polonium-210 serves as a reliable, compact neutron source for research and industrial gauging applications.
However, the element is notorious for its extreme radiotoxicity, making it one of the most dangerous substances known. Polonium-210 is an alpha-emitter; while alpha particles cannot penetrate human skin, they are devastating if the substance is ingested or inhaled. Once inside the body, the alpha particles release high energy over a short range, causing catastrophic damage to surrounding tissue and organs. Lethal doses are measured in micrograms, and the element is estimated to be hundreds of thousands of times more toxic than hydrogen cyanide by weight when internalized.