METALLOIDS: CHARACTERISTICS, PROPERTIES AND USES - CHEMISTRY - 2025

The metalloids or semimetals are a group of chemical elements with physical properties and chemical intermediate between metals and nonmetals. Most chemical researchers accept the following chemical elements as metalloids: boron, silicon, arsenic, germanium, antimony, and tellurium (green in the image below).

However, a smaller group of researchers add polonium, astate (blue in color) and selenium (pink in color) to metalloids.

Source: By Sandbh, from Wikimedia Commons

Even based on some properties, they suggest that the chemical elements carbon and aluminum (yellow in color) should also be considered as metalloids.

Main characteristics of metalloids

Situation on the periodic table

Metalloids are located on the periodic table on a downward diagonal between columns 13, 14, 15, 16, and 17, starting with boron at the upper left and ending with astatine at the lower right.

Metals are located to the left of metalloids and nonmetals to their right; therefore, they represent the border between both types of matter.

They form alloys with metals

Metalloids form alloys with metals and react with nonmetals, for example oxygen, sulfur, and halogens.

Electric semiconductors

For the most part they are considered as electrical semiconductors, their conductance being temperature dependent. At low temperatures the electrical conductance is low, so they act as electrical insulators, but as they heat up, their ability to conduct electricity increases.

Electronics Industry Base

Semiconductors are the basis of the development of the electronics industry, as well as that of computing and informatics. Likewise, the application that has been made of silicon has been very useful in this field.

Allotropic states

Metalloids have different allotropic states (different crystalline forms); thus, for example, arsenic has black, yellow or gray crystals.

In nature, they are not usually found as pure chemical elements, but rather associated or forming aggregates in minerals together with lead, sulfur, iron, etc.

Physical and chemical properties

Physical properties

They appear as shiny solids. In this respect they resemble metals. They are brittle and not very elastic, so they cannot be stretched into a wire, that is, they are not very ductile. In addition, its transformation into sheets is difficult, so the metalloids are not very malleable.

They are capable of conducting electricity and temperature, although to a lesser degree than metals. Within the metalloids there are chemical elements that, based on their band structures, are classified as semiconductors.

This group is made up of boron, silicon, germanium, and antimony. Arsenic and tellurium are classified as semi-metals.

Fusion Points

Boron 2,076 ° C; Silicon 1,414 ° C; Germanium 938.25 ° C; Arsenic 817 ° C; Antimony 630.13 ° C; Tellurium 449.51º C and Polonium 254º C.

Boiling points

Boron 3,927 ° C; Silicon 3,265 ° C; Germanium 2,833 ° C; Arsenic 614 ° C; Antimony 1,587º C; Tellurium 988º C and Polonium 962º C.

Densities

Boron 2.34 g / cm ³: Silicon 2.33 g / cm ³; Germanium 5.323 g / cm ³; Arsenic 5,727; Antimony 6.697 g / cm ³; Tellurium 6.24 g / cm ³ and Polonium 9.32 g / cm ³.

Chemical properties

They behave in a similar way to nonmetals, they form oxacids like SiO _2, and they have amphoteric behavior. Metalloids can behave like an acid or a base depending on the pH of the medium.

Applications

About living beings

-Arsenic is used in agriculture as an insecticide and herbicide. In addition, it is used to place it as a powder or in liquid solution on livestock to eliminate insects and parasites from the animal. Calcium arsenate is used to kill the cotton boll weevil.

-Arsenic is used as a wood preservative due to its toxicity to insects and fungi.

-Arsenic is used in the treatment of Acute Promyelocytic Leukemia, a type of blood cancer. It is used in the preparation of Fowler's solution for use in the treatment of psoriasis. A radioactive isotope of arsenic (⁷⁴ As) is used in the localization of cancerous tumors present in the human body.

-Arsenic is part of Melarsoprol, a medicine used in the treatment of Human African Trypanosomiasis. Parasitic disease transmitted by the tsetse fly.

-Tellurium oxide has been used in the treatment of seborrheic dermatitis. Likewise, other tellurium compounds are used as antimicrobial agents.

-Boron, in the form of boric acid, is used as a mild antiseptic in the eyes, nose and throat.

In glasses and enamels

-Tellurium is used in the production of blue, brown and red glasses. Metalloid can be electrolytically deposited on silver producing a blackish finish.

-Antimony is used to give glasses and enamels a yellow hue. Boron is used in the production of glass and ceramics. In particular, borosilicate glass is resistant to temperature changes, which is why it is used in laboratories in chemical reactions and distillations.

-In the home, food can be baked using borosilicate glass, without causing the utensils used to break.

-Silicon is the main base of the glass industry, involved in the manufacture of almost all glass objects.

-Germanium oxide is used in the manufacture of photographic camera lenses and microscope lenses. In addition, it is used in the manufacture of the core of optical fibers for many applications.

In the production of higher quality materials

-Arsenic forms alloys with lead, producing a decrease in its melting point. This causes a higher hardness in the alloy used in the production of shot.

-The addition of an amount of tellurium between 0.1% and 0.6% of an alloy with lead increases its resistance to corrosion and traction with an increase in flexibility. Tellurium is often added to cast iron to harden the surface layer of tempered parts.

-Antimony is used in alloys to produce bearings, accumulator plates and printing material.

-Silicon is used in the production of alloys with greater resistance to acids. Such is the case of Duriron, which contains 14% silicon.

The alloy of silicon, iron and aluminum is used for the production of very hard parts, which are used in the automotive industry.

-Arsenic forms alloys with platinum and copper to increase its resistance to corrosion. Also, arsenic is added to alpha-brass to increase the resistance of zinc. This type of brass is used in the production of accessory materials for plumbing.

In electronics and computing

-Metalloids are used as semiconductors in the electronics and computing industry. In this sense, silicon is the leader in the semiconductor trade that forms the basis of modern electronics and computing. Silicon and its derivatives are used in computers, transitors, solar cells, and LCD screens.

-Tellurium is a semiconductor that has applications in electro-optics and electronics.

-Germanium is a semiconductor metalloid that is used in conjunction with silicon in high-speed integrated circuits to improve performance. Although germanium has somewhat displaced silicon in its role as a semiconductor, its use in miniaturized chips has been enhanced.

-Germanium is used in the production of solar panels. Even exploring robots on the planet Mars contain germanium in their solar cells. Furthermore, germanium is used in the manufacture of radars.

Protective Action of Metalloids

Boron and its related compounds confer great resistance to the materials of which it is a part. This allows its use in the creation of spatial structures. Additionally, they are used in the production of golf clubs and fishing rods.

The protective action of boron carbide is used as control barriers in nuclear reactors, limiting the leakage of radioactive material. Furthermore, boron carbide is used in bulletproof vests and in the armor of war tanks.

Silicon dioxide and silica, in the form of clay or sand, are important components of bricks, concrete and cement, used in different forms of construction.

Others

-Antimony sulfide is used in fireworks and flash lamps in cameras.

-Boron is part of neodymidium magnets.

-Silicone, a polymer derived from silicon, is used in the production of oils and waxes, breast implants, contact lenses, explosives and in pyrotechnics.

-Germanium is used in the manufacture of fluorescent lamps and some LED diodes. Additionally, germanium is used in electric guitars to produce a characteristic distortion tone.

-Germanium is used in thermal imaging application for military use and fire fighting.

-Antimony is used in the manufacture of matches and tracer and locator grenades, as well as in primers for cartridges.

-Sodium borate is used as a combustion retardant in plastics and rubbers.

The 8 metalloid elements

This group of chemical elements is made up of boron, silicon, antimony, tellurium, germanium, arsenic, polonium, and astate. However, the largest number of researchers in the field of chemistry exclude polonium and astate as metalloids.

Therefore, the most widely accepted group of metalloids would be made up of boron, silicon, antimony, tellurium, germanium and arsenic.

Polonium has been noted to be distinctively metallic, since its two allotropic forms are metallic conductors. On the other hand, astatine was classified in 2013 as a metal, although previously in 1950 it was designated as a halogen, a reactive nonmetal.

The boundary between the groups of elements considered as metals, metalloids or non-metals is unclear. For this reason, some researchers, based on some property, suggest that this or that element should be considered as metalloid. It has been pointed out, for example, that carbon, aluminum or selenium should be classified as metalloids.

An attempt has been made to establish selection criteria that allow classifying a chemical element as metal, metalloids or non-metal. Other selection criteria include ionization energy, electronegativity, and packing efficiency of the different chemical elements.

References

1. Eden Francis. (2002). Classification of the Elements. Taken from: dl.clackamas.edu
2. Metals, Metalloids, and Nonmetals. Taken from: angelo.edu
3. Elements. Metalloids. Taken from: elements.org.es
4. Example of. (2013). Metalloids. Taken from: examplede.com
5. Wikipedia. (2018). Metalloid. Taken from: en.wikipedia.org
6. Chemistry Quick Fasts. (2011). Metalloids (Semimetals). Taken from: chemistry.patent-invent.com
7. The Editors of Encyclopaedia Britannica. (2016, October 18). Metalloid. Taken from: britannica.com