Metalloids - What They Are, Their Properties & Chemstries

Entry-level chemistry usually begins with the metals and the non-metals, especially the alkali metals and the halogens, specifically sodium and chlorine. These elemental substances are wildly different in their properties, as different as black from white. Yet there is a category of elements that smears the boundaries - the metalloids. Which among the elements are they and what are their properties, their uses and their chemistry?

Metalloids are elements that aren't quite metals, yet they resemble them enough that it isn't quite correct to call them non-metals, either. The metalloids lie in a zigzag strip between the metals and the non-metals in the Periodic Table of the Elements. They include boron, silicon, germanium, arsenic, antimony, tellurium and possibly polonium.¹

Unlike most metals, most metalloids are amphoteric-that is they can form both an acid and a base. For instance, arsenic forms not only salts such as arsenic trichloride, by the reaction

As₂O₃ + 6 HCl → 2 AsCl₃ + 3 H₂O,

but it also forms arsenites by reactions such as

As(OH)₃ + NaOH → Na(AsO₂) + 2 H₂O.

Most metalloids have a multiplicity of oxidation states or valences. For instance, tellurium has the oxidation states +2, -2, +4, and +6. Curiously, metalloids react like non-metals when they react with metals and act like metals when they react with non-metals.

Each of the metalloid elements has its own uses, and not all metalloids are used for the purpose, a number of them have been processed and doped to form semiconductors. It is from these semiconductors substances that diodes and transistors used in modern electronics have been made, enabling advances is communications, including especially in the realm of computers.

Antimony, in particular, has a plethora of individual applications. It is used in low-temperature and anti-friction alloys, low-temperature solders, electroplating baths as an additive, and in batteries. It is used to impregnate plastics and fabrics as a flame-retardant. It was formerly used extensively in the printing industry. It is used in fireworks, and as a pigment.

Interestingly, both boron and silicon are able to bond to other atoms of their own kind in much the same fashion that carbon does. This has made them useful in special ways, including the forming of inorganic and mixed organic-inorganic polymer materials. The word "silicones" will be familiar to the reader. Silicones are used as rubbers, resins, oils, and greases. Boron forms carbide hollow and threadlike nanostructures that appear promising.

Do other metalloid elements bond to themselves, and can they form polymeric substances? Yes, germanium, for one, forms polygermanes. As new metalloid preparations are increasingly developed, doubtless this fascinating class of compounds will assume a larger niche in the commercial world.

¹ Due to its scarcity, it has not been definitely ascertained polonium is a metalloid.

Resources:

NY Times - Theory and Experiment Meet, and a New Form of Boron is Found

University of Southern Mississippi, Polymer Science Learning Center - Inorganic Polymers

Mineral Information Institute - Antimony