The periodic table organizes all known elements into groups based on their shared chemical and physical characteristics. Elements are broadly categorized into three main groups: metals, nonmetals, and metalloids. Tennessine (Ts, atomic number 117) is one of the most recently synthesized superheavy elements. Its position in a group of classic nonmetals suggests its behavior may be radically different due to its extremely high atomic mass. The resulting debate centers on whether Tennessine should be classified as a metalloid, a metal, or a nonmetal, with the answer resting solely on theoretical predictions.
The Defining Characteristics of Metalloids
Metalloids are a small group of elements that exhibit properties positioned between those of metals and nonmetals. They are typically found along a “stair-step” line in the p-block of the periodic table, separating metallic elements from nonmetallic elements. Elements such as Boron (B), Silicon (Si), and Arsenic (As) are recognized members of this group. Physically, metalloids often display a metallic luster, appearing shiny like a metal, but they are typically brittle and shatter easily. Their most distinguishing feature is their intermediate electrical conductivity; they are not excellent conductors like copper, nor are they insulators like sulfur. This makes them semiconductors, a property that is foundational to modern electronics and technology. Chemically, metalloids can form compounds that show both metallic and nonmetallic tendencies.
The Unique Position of Tennessine on the Periodic Table
Tennessine is a synthetic element, meaning it must be created in a laboratory through nuclear fusion reactions. Scientists produce it by bombarding Berkelium-249 with Calcium-48 ions. With an atomic number of 117, Tennessine sits in Period 7 and belongs to Group 17, the family of elements known as the halogens. The lighter elements in Group 17, such as Fluorine, Chlorine, and Bromine, are textbook nonmetals that are highly reactive and form negative ions. Tennessine is positioned directly beneath Astatine, the heaviest naturally occurring halogen, which is already thought to exhibit some metalloid characteristics. However, the extreme instability of Tennessine is a major barrier to study, as its most stable isotope, Tennessine-294, has a half-life of only about 80 milliseconds.
Relativistic Effects and Predicted Properties of Tennessine
The immense atomic number of Tennessine (117 protons) introduces relativistic effects, which drastically alter its predicted properties. In extremely heavy atoms, the high positive charge of the nucleus accelerates the innermost electrons to speeds that are a significant fraction of the speed of light. This increase in electron speed leads to a corresponding increase in their mass, causing the electrons in the outer shells to behave differently than predicted by classical chemistry. Specifically, relativistic effects cause the outer electron orbitals to contract and split, fundamentally changing the element’s electronic structure. This results in Tennessine having a much lower predicted ionization energy compared to its lighter halogen counterparts, a property more typical of metals. Theoretical models suggest that Tennessine will not behave like a typical halogen, but rather will exhibit significant metallic character. It is predicted to be a volatile solid with a dark, metallic appearance and a relatively high boiling and melting point, potentially behaving more like a post-transition metal than a metalloid.
Why Classification Remains Unconfirmed
The classification of Tennessine remains purely theoretical because empirical study is currently impossible. The synthesis of Tennessine atoms occurs only one at a time, resulting in only a handful of atoms ever having been created. This minute quantity is insufficient to perform any direct physical or chemical experiments needed to measure its conductivity, density, or melting point. The half-life of 80 milliseconds for Tennessine-294 means that any synthesized atoms decay almost instantly. This time frame is far too short to allow the atoms to be collected or subjected to the chemical reactions necessary to determine its behavior. Consequently, all current consensus predicting Tennessine to be a metal is based entirely on complex computational chemistry models that incorporate these strong relativistic effects.