Metalloids

FIELDS OF STUDY: Inorganic Chemistry; Geochemistry; Metallurgy

ABSTRACT

The basic properties of the metalloid elements are discussed as they relate to the electronic structure of their atoms. The metalloids are also an essential component of semiconductors, and their use in modern digital electronics based on the transistor is their principal application.

The Nature of the Metalloids

The metalloids consist of a small group of elements that include germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te), boron (B), and silicon (Si). These elements are termed "metalloids" because they have some metallike properties, such as luster, but not sufficiently so to be classified as metals. For example, the metalloids do not conduct electricity well at room temperature, though they can be made to become conductive at higher temperatures or with small amounts of other elements in the lattices of their crystalline structure. The metalloids have intermediate electronegativity, so they are also neither as electronegative as the nonmetal group 16 and 17 elements (the chalcogen and halogen groups) nor as electropositive as the transition metals. Accordingly, the metalloids are characterized by intermediate ionization that typically falls between that of metals and nonmetals. The metalloids are not inert elements but quite reactive under the right conditions, depending on the properties of other elements in the reaction. Numerous compounds of metalloid elements are known, though the metalloids are most often used in their elemental form as components of alloys, in which they enhance the ductility and malleability of the particular combination of materials.

Electronic Structure of the Metalloids

The electrical conductivity of metalloids can be described in terms of molecular orbitals, which form from the combination of and interactions between the atomic orbitals of bonded atoms. In a simple bond between two atoms, two molecular orbitals of different energy levels are formed. As more atoms are bonded together in a molecule, an increasing number of atomic orbitals become available to form molecular orbitals, and the resulting energy levels become more closely spaced, ultimately forming a continuum of unoccupied energy levels called a "band." The large number of atoms in a macroscopic quantity of metal form an equally huge number of molecular orbitals, creating a tightly spaced band of orbitals that allows electrons to move more easily between the various energy levels. In metals, the presence of empty molecular orbitals (called the "conduction band") that are close in energy to filled molecular orbitals (the valence band) enables the electrons to become highly mobile and delocalized from the atom, which facilitates the flow of a current through a material. In a metalloid, there is a larger energy gap between the valence band and the conduction band than in metals, which prevents electrons from flowing easily through the conduction band. However, some electrons naturally acquire enough energy to "jump" into the conduction band. Accordingly, the electrical conductivity of the metalloids increases when temperature is elevated, as more electrons are able to acquire the energy needed to move into the conduction band. Likewise, the conductivity of the metalloids decreases relative to metals when the temperature decreases.

Metalloid Compounds

All of the metalloids form compounds according to the number of valence electrons they have, but their bonding tends to be more covalent than ionic in character. Germanium, for example, forms compounds with the halogens and oxygen by sharing its four valence electrons rather than by giving them up to form ions. This permits the recovery of the metalloids in extremely pure form from the minerals and inorganic sources, such as coal ash, in which they naturally occur. Germanium ore is first reacted with hydrochloric acid (HCl), producing the volatile liquid germanium tetrachloride (GeCl₄) as an intermediate. Germanium tetrachloride can be hydrolyzed to form germanium dioxide (GeO₂), which is then reduced to elemental germanium using hydrogen gas (H₂). Zone refining, or melting, of the material yields germanium with just one part of impurity in 10¹⁰, or 99.9999999 percent purity. Of the metalloid elements, germanium poses no health risks, but antimony can form the toxic compound stibine (SbH₃) by reaction with strong acids. All compounds of arsenic and tellurium are highly toxic.

Applications of the Metalloids

The semiconducting properties of the metalloids, particularly germanium and silicon, have enabled the development of solid-state electronics, such as semiconductor diodes, integrated circuits, light-emitting diodes (LEDs), and liquid-crystal displays (LCDs). Semiconductors are essential components of transistors, and the production of computer chips is the primary use of germanium and silicon. The complex process begins with the growth of large single crystals of silicon or germanium from the molten state. In order to improve the conductivity of these semiconducting elements, which both have four valence electrons, a small amount of another element, called a "dopant," is introduced into the silicon or germanium lattice. Dopants for silicon and germanium are typically group 3 or group 5 elements (which have three and five valence electrons, respectively), thereby increasing the number of charges that can move through the lattice and greatly enhancing the material’s conductivity. The doped semiconducting crystal is then sliced into wafers and subjected to numerous etching processes that assemble minute transistor structures on its surface. These are the heart of all digital devices.

PRINCIPAL TERMS

• alloy: a mixture of a metal and at least one other element, often another metal; also known as a solid solution.
• amphoteric: describes a compound with the ability to act as either an acid or a base, depending on its environment and the other materials present.
• conductivity: the ability of a material to transfer heat (thermal conductivity) or electricity (electrical conductivity) from one point to another.
• ductility: the ability of a solid material to be deformed by the application of tensile (pulling) force, such as bending, without breaking or fracturing.
• malleability: the ability of a solid material to be deformed by the application of compressive (pushing) force, such as hammering, without breaking or fracturing.

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