Metalloids

Metalloids are a group of seven elements found between the metals and nonmetals on the periodic table. Metalloids share some characteristic properties of metals, such as metallic luster. They also share properties of nonmetals: they generally do not conduct electricity as well as metals and they tend to be brittle. Metalloids have electronegativity values that range between those of metals and nonmetals. This intermediate electronegativity is the reason that metalloids behave similarly to both metals and nonmetals.

Silicon and boron exhibit characteristic bonding behavior and have many practical applications due to the way they interact with oxygen. A silicon-silicon bond is not particularly strong. On the other hand, a silicon-oxygen bond is quite strong. A mineral that contains both silicon and oxygen is a silicate. Silicates can grow to be enormous molecules by alternating the silicon and oxygen atoms. Rocks, sand, and clay are mostly made up of silicates. Glass is another example of a silicate.

A solid material with more than one possible structure is a polymorph. An example of a polymorph is silica. It forms the basis of many common minerals and can appear in different forms. Both sand and quartz are minerals that are made mostly of silica. The orthosilicate ion (SiO₄^4–) is the basis of many silicate minerals. Orthosilicate has a tetrahedral shape consisting of one silicon atom and four oxygen atoms. Two orthosilicate ions can be connected by an oxygen atom, forming a disilicate ion (Si₂O₇^6–). The disilicate ions can be connected to each other to form a chain, a sheet, or a three-dimensional shape. When each orthosilicate ion connects to four other orthosilicate ions, a three-dimensional chain solid is formed. Because each orthosilicate is connected to four others by an oxygen atom, the net formula of the compound becomes silica (SiO₂). Silica can be melted, which breaks down many of the silicon-oxygen bonds, and made into glass. But if the liquid is cooled too rapidly, the atoms cannot order themselves into a proper crystalline structure. The result is an amorphous solid, a solid composed of particles that are not organized into a crystalline pattern. Another metalloid is boron. Boron is used to change properties of other materials and is used as an additive in fiberglass and ceramics. Pure boron has two forms. It can be either a solid hard crystal or amorphous brown powder at room temperature. Boron has three valence electrons. Its oxidation state is +3 in most stable compounds. Boranes are compounds that contain only boron and hydrogen. When boron bonds with three hydrogen atoms borane (BH₃) is formed. This compound has six valence electrons. Boranes can further react with another borane to form diborane (B₂H₆). In diborane, two of the hydrogen atoms form a bond with each boron atom, bridging them. A borate is an ion with boron-oxygen bonds. The simplest borate is orthoborate ([BO₃]^3– ). More complex ions have multiple boron and oxygen atoms. A tetraborate anion, for example, has the formula [B₄O₇]^2–. Borax, the most commonly used borate salt, has the formula Na₂B₄O₇·10H₂O. Boron occurs naturally and can be found in dry lake deposits where lakes have had repeated evaporation. The boron is mined from these deposits and is often used to prepare other boron compounds.