Polonium (Po), element 84, is classified based on its chemical and physical behaviors, which place it among metals, nonmetals, and metalloids. Polonium’s classification has been a source of frequent debate because its properties do not fit neatly into any single group. The confusion stems from its location near the dividing line between metals and nonmetals, combined with its highly distinctive nuclear nature. Understanding Polonium requires a look at the element’s unusual characteristics.
Defining the Metalloid Category
Metalloids are elements possessing properties that fall between those of typical metals and nonmetals. They are found along a diagonal “stair-step” line on the periodic table, separating the highly conductive metals from the insulating nonmetals. This intermediate position makes them technologically valuable.
A defining physical feature of metalloids is their moderate and conditional electrical conductivity; they function as semiconductors. Their ability to conduct electricity can be precisely controlled by temperature or by adding impurities. Common examples like Silicon and Germanium are the basis for nearly all modern electronic devices because of this property.
Physically, metalloids often exhibit a metallic luster, appearing shiny and reflective, yet they tend to be brittle rather than malleable or ductile like true metals. Chemically, they often display amphoteric behavior, meaning their oxides can react with both acids and bases. This differentiates them from the strictly basic oxides of metals and the acidic oxides of nonmetals. They also tend to form covalent bonds, a trait shared with nonmetals.
Polonium’s Classification Debate
Polonium’s position in Group 16, below the metalloid Tellurium, leads to its frequent misclassification as a metalloid. However, modern consensus generally classifies Polonium as a poor metal or post-transition metal, based on a preponderance of metallic characteristics. Its appearance is a lustrous silvery-gray solid, and its structure is a simple cubic lattice, a form found in some metals.
Polonium’s electrical conductivity increases as temperature decreases, a behavior typical of metals, whereas metalloids acting as semiconductors show the opposite trend. It also readily dissolves in dilute acids, forming the rose-colored Po²⁺ cation and displacing hydrogen, a chemical reaction characteristic of metals. Furthermore, Polonium forms a dioxide (PoO₂) that is predominantly basic, which is more typical of a metallic oxide than the amphoteric or acidic oxides formed by true metalloids.
Polonium possesses some properties that hint at a nonmetallic nature, such as forming unstable compounds called polonides, which contain the Po²⁻ anion. Despite these few borderline traits, the weight of evidence—including its metallic crystal structure, electrical behavior, and chemical reactions with acids—places Polonium firmly among the post-transition metals. Its proximity to the metal-nonmetal boundary is the primary reason for the enduring confusion.
The Highly Radioactive Nature of Polonium
Polonium’s extreme radioactivity sets it apart from nearly all other elements. Polonium was discovered in 1898 by Marie and Pierre Curie, who named it after Marie’s native land, Poland. The most studied isotope, Polonium-210 (\(^{210}\)Po), has a short half-life of only 138 days.
This rapid decay process releases a tremendous amount of energy, making Polonium-210 an extremely potent alpha emitter. The high specific activity of the isotope causes it to generate significant heat, so much so that a milligram of pure Polonium-210 can self-heat to high temperatures. This inherent thermal and nuclear instability makes it exceedingly difficult to study its basic chemical properties without the interference of radiation damage.
The alpha particles emitted by Polonium-210 are highly damaging to biological tissue if the substance is ingested or inhaled. While alpha radiation cannot penetrate the outer layer of skin, making external contact relatively safe, Polonium-210 is considered one of the most radiotoxic substances known. A minuscule amount, estimated to be around 10 micrograms, can be a lethal dose for a human, making its toxicity hundreds of thousands of times greater than that of cyanide.