By Ron B. Davis Jr., Georgetown University
The scientific community has decided that metalloids are those elements with properties that are intermediate between those of metals and nonmetals. Because of the ways the metalloids combine, they can be considered as the chameleons of the periodic table—elements whose properties can appear metallic in one environment, but decidedly nonmetallic in others.
Arsenic’s Toxicity
The metalloid arsenic’s compounds such as arsenic sulfide and arsenic oxide minerals have probably been known since pre-history. Arsenic’s famous toxicity comes at least in part from its ability to impersonate its nonmetal sibling phosphorus. When arsenic gets into the human body, it disrupts essential phosphorus-related biochemical processes, acting as a poison to every cell in our body.
This once made arsenic the poison of choice for people, at least until 1836. That was when chemist James Marsh developed a simple test to detect even small quantities of arsenic. All it took was metallic zinc, sulfuric acid and a flame.
Zinc is much higher on the activity series than arsenic. So, Marsh’ reaction strips electrons away from arsenic while also producing hydrogen gas from the acid. The arsenic and hydrogen then react to form arsine gas. As the periodic table predicts, this compound has a formula of AsH3, the same valence and bonding arrangement as NH3, formed by arsenic’s fellow group member nitrogen when it makes ammonia.
When burned, arsine gas produces a distinctive purple flame, and also pure arsenic residue. This residue can be collected by placing a cold piece of glass into the flame, where it produces a dark deposit of arsenic. This deposit could be preserved and used in investigations and legal proceedings involving suspected arsenic poisoning.
Adding Arsenic to Metallic Substances
But arsenic is much more than just a poison. It can also be added to metallic substances to alter their properties. Since ancient times, the addition of just a few per cent of arsenic by mass to copper has been used to create a material known as arsenical bronze. In this material, arsenic is incorporated into the metallic structure of copper, providing a superior material for the production of cutting tools.
The presence of arsenic in arsenical bronze helps to disrupt the highly ordered structure of copper’s metal atoms. When arsenic atoms are incorporated into the structure of copper metal, their different sizes break up certain planes of symmetry in the material, making it harder for atoms of the new material to slide past one another during deformation. In this material, arsenic acts a great deal like the metallic element tin, which is often used to strengthen copper in the production of traditional bronze.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Metalloid Antimony
Just below arsenic on the table is antimony, an element that takes its name from the Greek terms meaning ‘not found alone’. As its name implies, antimony is commonly found mixed with other elements. In ancient times, antimony was often encountered as antimony sulfide, while antimony oxide was added to plastics as a flame-retardant until concerns arose about toxicity.
Just like the other metalloids, antimony’s behavior ranges from very metal-like to very nonmetal like.
When mixed with the nonmetal halogens, antimony can form a range of nonmetal compounds. Many of these antimony compounds exist as discrete molecules with covalent bonds, like antimony trichloride, also called butter of antimony. In this form, antimony is sometimes included in fire-retardant chemical preparations.
Yet when antimony is mixed with metals, it forms alloys, acting like a metal. Being incorporated into the metallic bonds of lead, for example, it can significantly improve the mechanical strength of this very dense metal, making better materials for applications from battery plates to boat anchors.
Properties of Metalloids
Metalloids represent a distinctive boundary area between the electron-hungry, covalent-bonding nonmetals and the electron-rich metals that form bands of electrons in metallic bonds.
Being in this transitional region of the p-block gives pure metalloids three sets of behaviors. First, in some ways, they seem like metals, for example; their shiny appearance. Second, other properties seem to be middle-of-the-road between metals and nonmetals—like their tendency to be semiconductors of electricity. Third, in other ways they more emulate nonmetals, as when tellurium on the early Earth made discrete molecules of gaseous tellurium hydride and then vanished into space.
Toeing the line between metal and nonmetal also gives these elements the remarkable ability to take on the characteristics of metals or nonmetals. Arsenic mixed with the metal copper forms a metallic alloy, but arsenic in combination with nonmetals forms small covalent molecules, like the fire-smothering butter of antimony or the arsine gas used to detect arsenic poisoning.
Additionally, substitution of one metalloid for another can dramatically improve the qualities of materials that we use every day. The conductivity of silicon and germanium can be fine-tuned using other metalloids in semiconductors, and a similar substitution happens also in borosilicate glass, which introduces boron in place of some silicon to create a glass material with properties perfectly suited to uses in the lab or kitchen.
Common Questions about Characteristics of Metalloids
When arsenic gets into the human body, it disrupts essential phosphorus-related biochemical processes, acting as a poison to every cell in our body.
Since ancient times, the addition of just a few per cent of arsenic by mass to copper has been used to create a material known as arsenical bronze. In this material, arsenic is incorporated into the metallic structure of copper, providing a superior material for the production of cutting tools.
The metalloid antimony takes its name from the Greek terms meaning ‘not found alone’. As its name implies, antimony is commonly found mixed with other elements.
