The term “metalloid” refers to chemical substances having properties range between those of metallic and nonmetallic elements. There is no universal definition of metalloids, and there is disagreement over which elements fall into this category. Their crystal structures are made of covalent bonds, making them very similar to nonmetals. These likewise have certain features with nonmetallic, such as the fact because these are not brittle. Consequently, they are important in electronic devices. Metalloids can be found in all organic and inorganic compounds. These chemicals have comparable properties with one another. They may be found on the periodical table’s rights edge, in the middle of the metallic and nonmetallic groupings. The only exception is hydrogen, which is a non-metal. The term metalloid was originally used to describe nonmetals, but the term has been used more recently to describe a group of intermediate elements (Harper et al., 2015).
1. What are metalloids?
Metalloids are an unusual class of chemical elements. Those solids are excellent heat conductors and electrical conductors. This property makes them useful as computer chip materials. During chemical reactions, they behave like semiconductors and can either take or lose electrons from other elements (Gong, Zhao & Wang, 2018).
1.1. Behavior
Although they resemble metals in appearance, metalloids behave more like nonmetals in most chemical reactions. They are less malleable than metals. They can act as metal alloys and flame retardants, and can be used to make steel alloys. Despite their intermediate properties, metalloids have numerous applications in chemical and technological fields, including as catalysts, optical storage media, and biological agents.
1.2. Density
Metalloids can vary in density. A metalloid’s density depends on its composition. Its density varies between nonmetals and metals, though it has a lower density than nearby metals. When the density of a metalloid materials is increased, its atomic radius also rises. This indicates that the outside electrons are farther from the nuclei and have a lower level of attraction to it. As a result, ionization energy decreases and electron loss increases.
Figure 1: Metalloids
2. Elements belonging to the group of metalloids
Elements belonging to this group are Boron, Silicon, Germenium, Arsenic, Tellurium and Antimony, polonium and astatine. Description of some important metalloids is given below (Li et al., 2021).
2.1. Germanium
Germanium is a kind of metalloid which can be found in the periodic table and is its own distinct scientific constituent. Its density is 5.323 g cm-3 and its abundance is about seven parts per million. Its most abundant mineral is germanite, which contains eight percent of the element. On the other hand, the vast majority of the global production of the element is mined from zinc ores, in which it is created throughout the beneficiation of zinc. The United States has two mines that produce germanium, while other nations also import it. Prior to its use in electronics, germanium had little use in the world. However, with the development of electronics, it started to prove its worth as a semiconductor.
2.2. Arsenic
Arsenic is a metalloid that can be found in a number of forms in nature. Its common form is a grey, shiny substance that is highly toxic and has been used as a rat poison. However, humans require trace amounts of arsenic for healthy growth. Arsenic is present in nature in oxidation states of -3, 3 and 5. This chemical element is found in a variety of chemical compounds.
2.3. Tellurium
Tellurium seems to be a factual basis that may be found in the periodic table and is classified as a metalloid. It has important applications in the rubber and textile industries. It is used as a catalyst in the vulcanization process, which transforms soft rubber into a harder product. It is also used to increase the strength of steel and lead alloys and to reduce the corrosive effects of sulfuric acid. Tellurium is a relatively rare chemical element. It is similar to tin, but has intermediate properties. When burned, tellurium emits a blue-green flame and a white gas known as tellurium dioxide. Tellurium is a silvery white metalloid that is brittle and corrosive. It can also corrode metals in a molten state.
2.4. Antimony
Antimony is a kind of metals that’s also classified under the category of metalloids. It has atomic number 51 and occurs naturally as a sulfide mineral called stibnite. Its chemical symbol (Sb) comes from the Latin word stibium, which means “not alone”. Antimony exists in the Earth’s crust at a concentration of 0.2 ppm. The main ore that contains antimony is the gray sulfide mineral stibnite. Antimony is widely shown in alloys to strengthen the tenacity and endurance of various metallic; nevertheless, this just also extremely hazardous when consumed through excessive quantities. It is also used in semiconductors. It was also used by the Byzantine Greeks in the 13th century for making “khol,” a type of mascara.
2.5. Polonium
Polonium being highly radioactive elements that is denoted by the symbol Po in the periodic table. It is a chalcogen, which means it is rare and highly radioactive. It also belongs to group of metalloids. Due to the absence of any stability isotopes, polonium is a substance that should be approached with caution. It is chemically similar to selenium.
2.5.1. Neoplastic changes
Animal studies on polonium exposure have shown it to cause neoplastic changes in rats. Of 175 animals injected with a single dose, 40 soft-tissue tumors developed in comparison to only three tumors in the control group. Most of these tumors were primary and malignant. The incidence of tumors was highest at the middle dose (20 uCi/kg). In contrast, the lowest dose (1 uCi/kg) resulted in fewer tumors.
2.5.2. Half-life
The half-life of polonium-210 is 138 days, which makes it relatively stable and easy to transport. On the other hand, it becomes exceedingly poisonous at large quantities, therefore it must be administered with the utmost care. It has the potential to result in the breakdown of several organs in addition to acute radioactive sickness.
2.6. Astatine
Astatine is one of the elements that is used in radiation therapy. Its radioactive isotope, astatine-211, decays rapidly and emits alpha particles. These particles are more powerful than beta particles and can kill tumor cells. They are also less toxic to normal tissue, which is important for radiation therapy.
2.6.1. Occurrence and discovery
As a radioactive element, astatine has a half-life of approximately eight hours. It’s found in the Continental mantle in minuscule amounts and ranks by most the rarest elements. It can be synthesized by bombarding bismuth with energetic atomic particles. This element was discovered by scientists Dale Corson, Kenneth McKenzie, and Emilio Segre in the 1940s.
2.6.2. Isotopes.
Astatine has 33 known isotopes. So every isotope has its own unique mass number, where it is a numerical representation of the total amount of electrons, protons, and neutrons contained inside an atoms. The most stable isotope is 210At, while the least stable is 213At, with a half-life of 125 nanoseconds.
2.7. Silicon
Si stands for the scientific element “silicone,” which has the atomic number 14. It’s a metallic-looking, blue-grey crystallized solids. It is a tetravalent metalloid and a semiconductor. Silicon is in group 14 of the periodic table, above germanium and tin and below lead.
2.7.1. Tetravalent
Silicon, the 8th most ubiquitous element in the existence, is a crucial component in several technological advances. Its high degree of semi conductivity makes it useful for semiconductors and electronic devices.
2.7.2. Crystalline and amorphous
Silicon is found in nature in two forms. It is found in a crystalline solid and as an amorphous brown powder. It is insoluble in acids, but dissolves easily in hot caustic alkalis. It also forms a protective oxide layer in the air.
2.8 Boron
Boron seems to be an element that shares many of the chemical characteristics of carbon along other elements. It can be obtained in many forms and can be used in various industries. In the 18th century, the elements boron and sodium were first discovered by French chemists Louis-Jaques Thenard and Joseph-Louis Gay-Lussac. A century later, English chemist Ezekiel Weintraub isolated boron in a more refined form. Boron is non-reactive with water, hydrochloric acid, and nitric acid. However, if exposed to hydrogen and nitric acid, boron will oxidize to a form called boric oxide. Boron compounds are being investigated for a wide range of applications.
3. Properties of metalloids
Metalloids are an important class of elements that possess several properties that differentiate them from nonmetals. These compounds are distinguished by their different behavior in chemical reactions, as well as their different types of bonding. Other metalloids may have four or more electrons, which make them act like metals or nonmetals depending on the other elements in the compound. Important properties of metalloids are given below (Harper, et al., 2015).
3.1. Chemical reactions
Metalloids are chemically similar to metals but have different properties. Their properties include reactivity and chemical behavior. Boron has three valence electrons. Boron behaves like a metal during chemical reactions, while other metalloids with four or more valence electrons behave like nonmetals.
3.2. Ductility
Metalloids are quite a group of chemicals having distinct metals and non-metallic allotropically forms. Their ductility differ greatly from those of other compounds. Additionally, it is known that they may couple using others elements, like halogen’s, to produce new combinations.
3.3. Conductivity
Another important property of metalloids is that they can be conductors of electricity under certain conditions. This property is known as semi conductivity. They do not have the p-valence-electron properties of metals, but do have a few properties of nonmetals. Usually they are found in zigzag structures and are found in relatively small quantities.
3.4. Toxicity
Toxicity of metalloids is often associated with non-essential elements, but can also arise in foods. Their use in agriculture and the environment can lead to severe contamination. Particularly toxic are arsenic and antimony compounds, although boron and silicon are essential trace elements. Arsenic is also known to be toxic to plants. Metalloids are found in soil at a wide range of concentrations, and their effects vary according to the pH of the soil.
3.5. Brittleness
Metalloids are brittle and can break into small pieces. This property makes them useful for alloying purposes. However, their brittleness makes them difficult for structural applications .Metalloids are brittle because their valence shells are not filled to the brim. This makes them difficult to withstand compression without breaking into small pieces.
3.6. Hardness
Metalloids are found in the p block and have a range of hardnesses. Unlike metals, metalloids are largely inert at room temperature. In contrast, metals typically ductile that stretch when under compression. Consequently, there is a greater potential for energy exchange involving respective atoms.
4. Uses and examples
Metalloids are relatively common and are used in everyday life. As a result, they are frequently included into alloys of other metals. However, their brittleness prevents them from being used in structural applications. Instead, they are used in other products, such as biological agents, optical storage, and pyrotechnics. For instance, the word “flint’s” in Latina is silica, which translates to “silicon’s,” which is one of the most abundant metalloids. This element has numerous applications, ranging from electronics to cosmetics (Gong, Zhao & Wang, 2018; Li, et al., 2021)
4.1. Antimonial lead alloy
Metalloids vary in shape and color and can be transparent or opaque. They can also be introduced into other metals to form alloys. Antimony is mixed with lead to form an antimonial lead alloy, which is used in ammunition. Antimony has an electronegativity value of 1.8 and can be mixed with lead to form an antimonial lead alloy.
4.2. Electronic
Metalloids have important applications as semiconductors Tellurium, for instance, is a metalloid that has many uses. In electronics, it acts as a semiconductor and lends strength to metal alloys. It is also used in cable and plumbing equipment. It is even a source of power in semiconductor devices.
4.3 Medicinal uses
Metalloids are the elements that have both medicinal and toxic properties. Examples of these include antimony and arsenic. Other metalloids that have medicinal uses include silicon and boron. These compounds are used in medicine to treat a range of ailments. Boron is a widely used antifungal and antiseptic, while silicone gel is used for skin infections. Arsenic, antimony, and tellurium are used for a variety of purposes and can be harmful in high doses.
Conclusion
“Metalloid” represents chemical elements with metals as well as nonmetals attributes. This category does not have a standard definition and there is no complete agreement about which elements are metalloids. Nonetheless, we can consider these elements as one of the most interesting classes of elements because of their unique properties. Although metalloids are nonmetals, they have metallic properties and can form metallic alloys. Their chemical and physical properties are intermediate, which makes them useful in alloys and other chemistry. Despite this, they have limited structural applications. However, they are used in biological agents, flame retardants, glass, optoelectronics, and pyrotechnics. Although not metallic, metalloids include the following elements: Boron, germanium, and silicon. Boron and silicon are the most commonly used semiconductor materials. Boron and tellurium are the two most toxic metalloids and were used in early medicine and art until their toxic properties became known.
References
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Gong, Y., Zhao, D., & Wang, Q. (2018). An overview of field-scale studies on remediation of soil contaminated with heavy metals and metalloids: Technical progress over the last decade. Water research, 147, 440-460.
Li, Y. P., Ben Fekih, I., Chi Fru, E., Moraleda-Munoz, A., Li, X., Rosen, B. P., … & Rensing, C. (2021). Antimicrobial activity of metals and metalloids. Annual Review of Microbiology, 75, 175-197.