The elements on the periodic table are broadly separated into metals, nonmetals, and a unique transitional group known as metalloids. This classification is based on a collection of physical and chemical properties, including their mechanical response to force. Metalloids possess a blend of characteristics from both metals and nonmetals, which makes their physical behavior complex. Examining the physical property of malleability helps determine where these “in-between” elements truly fall.
What Malleability Means
Malleability is a physical property that describes a material’s ability to deform plastically under compressive stress without fracturing. In simpler terms, a malleable material can be hammered, pressed, or rolled into thin sheets without cracking or breaking apart. This property is closely related to ductility, which is the ability to be stretched into a wire under tensile stress.
The mechanism for malleability is rooted in the internal atomic structure of a material. Metals are held together by metallic bonds characterized by a “sea of electrons” that are delocalized and shared among all the atoms. This non-directional bonding allows layers of atoms to slide past one another when a force is applied without disrupting the overall chemical bond. When the force is removed, the atoms remain in their new, deformed positions, resulting in a permanent change in shape.
Defining the Metalloid Class
Metalloids, also known as semimetals, are elements like Boron, Silicon, Germanium, Arsenic, Antimony, and Tellurium that fall along the zigzag “staircase” line separating metals from nonmetals. Their intermediate position means they share traits with metals (like metallic luster and moderate electrical conductivity) and nonmetals. This dual nature makes them valuable in modern electronics as semiconductors.
Their intermediate nature results from their unique bonding structure, which differs significantly from the delocalized bonds of metals. Metalloids primarily form covalent crystal structures, similar to nonmetals. This involves the sharing of electrons between neighboring atoms in a fixed, localized, and highly directional manner. This directional covalent bonding dictates their mechanical response to physical stress.
Why Metalloids Are Generally Brittle
The difference between metallic bonding and the covalent bonding found in metalloids explains their lack of malleability. Metalloids rely on fixed, directional covalent bonds to form their crystal lattice, meaning compressive stress cannot be accommodated by atomic layers sliding easily. The bonds are rigid and localized to specific neighbors, preventing them from reforming in a new position.
When a force is applied, these strong, directional bonds resist deformation until the stress concentration becomes too high, at which point the bonds break abruptly. This failure results in brittleness, causing the material to shatter or crumble instead of deforming into a new shape. Crystalline silicon, a widely used metalloid, is known to be very brittle and will fracture easily when struck. Metalloids are therefore classified as brittle solids, a characteristic they share with nonmetals, despite their shiny, metallic appearance.