Metalloids or semimetals, refer to a group of elements that lie between metals and nonmetals. On the periodic table, they lie between nonmetals and poor metals. Metalloids show characteristics of metals and nonmetals because of their position on the periodic table.
Also, they have characteristics of metals and non-metals because they have partially filled outer electron shells. As I discuss the characteristics of metalloids further in this article, you will get to understand the uniqueness of this class of elements by considering the elements individually.
What are metalloids?
Metalloids are a group of elements with properties that border between metals and nonmetals. They have a metallic appearance but behave like nonmetals. Elements in this class are boron, silicon, germanium, arsenic, antimony, tellurium, and polonium.
Elements of metalloids
Boron
Boron, represented by the letter “B”, is the fifth element of the periodic table and the first in the family of metalloids. It has an atomic number of 5 and a mass number of 11. Boron exists in several allotropes – amorphous and crystalline boron.
Amorphous boron is a brown powdery solid while crystalline boron is a silvery to black hard solid. Boron does not occur freely in nature. It exists as orthoboric acid, borates, and crystals of boron carbide and boron nitride.
Silicon
Silicon (Si) is the most common metalloid, and it finds the most application. It is the second most abundant element in the Earth’s crust and is extremely reactive in its pure form.
It has an atomic number of 14 and a mass number of 28. Silicon is hard and brittle with a blue-grey metallic luster. It exhibits the tetravalency of carbon and is commonly found as silicon dioxide (silica).
Germanium
Germanium (Ge-32) is a glossy, hard, brittle, element with a greyish-white luster. If you are not very familiar with the metalloids, you might mistake germanium for silicon in appearance. It also has several similarities with silicon and tin.
The naturally occurring germanium compounds are insoluble in water, but this element finds several applications in the electrical and light industries.
Arsenic
Arsenic, As, has an atomic number of 33 and a mass number of 75. It exists in pure form but often occurs in combination with sulfur and some other metals. Arsenic also has several allotropes like boron, but the gray form is the most common.
Antimony
Antimony, Sb, is a glossy gray metalloid that naturally occurs as a sulfide mineral stibnite. It has an atomic number 51 and a mass number 121.75. Antimony compounds have various applications across different industries.
Tellurium
Tellurium is the sixth metalloid. It has an atomic number of 52 and a mass number of 127.6. It is a silvery white brittle metal that is slightly toxic. Tellurium is available in trace amounts on the Earth’s crust but can be found in the universe.
It has the potential to form a volatile hydride which evaporates as a gas during hot nebular formation on the earth.
Polonium
Polonium is a radioactive element, which is why it is sometimes classified as a radioactive element. It has an atomic number of 84 and a mass number of 209. It is a silvery gray or black element that belongs to the oxygen group.
Physical properties of metalloids
- Metalloids are solid, brittle metals at room temperature
- They have the shiny, reflective appearance of metals
- Although metalloids share characteristics with metals, they are not as ductile which makes them unsuitable for structural applications
- They do not have a fixed density. Density varies with metalloids. They are less dense than metals but denser than nonmetals
- The boiling and melting points of metalloids intermediate between that of metal and nonmetals. Their melting and boiling points are lower than most metals but higher than nonmetals
- Metalloids have a fair conductivity for electricity and heat. They behave as semiconductors
- Most metalloids exhibit allotropy
Chemical properties of metalloids
- Metalloids are fairly electronegative. They often have electronegativity values between 1.8 and 2.2
- Because of their intermediate electronegativities, they tend to exhibit both negative and positive oxidation states in their compounds
- Also, metalloids form covalent bonds
- They are very reactive and can gain or lose electrons to form anions or captions, respectively
- They readily react with nonmetals to form compounds
- Metalloids readily mix with other metals to form alloys
- In addition, the reaction between metalloids and acids varies. For instance, silicon, germanium, and polonium are insoluble so they do not react with acids. However, arsenic and antimony react with strong oxidizing acids
Applications of metalloids
These are the applications of each metalloid:
Boron
- Boron, in the form of borax oxide, is mixed with silicon, in the form of silica sand (SiO2), and soda ash to form borosilicate glass. Borosilicate glass is the raw material used to make Pyrex bakeware because of its high resistance to thermal shock
- It is a good alloy metal. Boron is added to steel alloys to increase hardness and strength.
- Boron fibers are very light but strong and find application in aerospace
- Boron carbide is a hard compound used in making armor tanks and protective gear
- Boron also has medicinal properties and is used in the production of antiseptics, herbicides, insecticides, and anti-foaming agents
- Boric acid, a compound of boron, is used in the treatment of yeast infections
- In addition, boron is often present in fertilizers used to improve plant growth and crop yield. It enhances hardness in cell walls, pollination in fruit plants, and builds plants for drought tolerance
Silicon
- Silicon semiconductors are used in cellphones, computer chips, solar cells, integrated circuits, computers and automotive electronics
- Polymerization of silicone results in a silicone-oxygen backbone that is used to produce silicone rubber
- Silicone is used as a lubricant, insulator, sealant, and manufacture of the handles of cooking utensils
- Silicon is an alloying metal. It is added to aluminum alloys to enhance their strength
- It is also used in the production of glass and ceramics
- Silicon is often mixed with phosphorus and is added to stoneware and ceramic glazes
- Silicon is also used in the production of antiseptics and anti-foaming agents, like boron
- Just like boron, silicon is also added to plant growth fertilizers. Additionally, adding silicon-based fertilizers to rice farms increases stem strength and resistance to pathogens
Germanium
- Like silicon, germanium is used as a semiconductor in computer chips, solar cells, and integrated circuits
- Germanium compounds are strong polymerization catalysts
- Other applications of germanium include solar cell applications, infrared optics, fiber optic systems, and light-emitting diodes
Arsenic
- Arsenic is added to lead alloys, such as automotive batteries and ammunition
- It is also used in the manufacture of pesticides, herbicides, insecticides, and treated wood products
- Some bacteria species use arsenic compounds as respiratory metabolites
Antimony
- Antimony is added to lead and tin alloys which are used in battery grids and solders. Adding antimony enhances the properties of solders, bullets, and plain bearings
- Antimony trioxide, a compound of antimony, is often added to plastics, textiles, coatings, and building materials as a flame retardant to improve fire safety.
- Powdered antimony compounds find use in medicine and cosmetics
Tellurium
- Tellurium compounds are used in rubber, pigments, explosives, slips, mold releases, and petroleum refining
- It has no biological role so far
- Tellurium is used to vulcanise rubber, tint glass, and ceramics, in solar cells, and as a catalyst in oil refining
- It can also be doped with silver, gold, copper, or tin in semiconductor applications
Polonium
- As a natural alpha-emitter, polonium is used as an alpha particle in the form of a thin film in a stainless steel disc
- Polonium is used as a heat source for space equipment. This is possible because as it decays, it releases high amounts of energy into its surroundings
- Polonium is also used to reduce static electricity during rolling
- It is used to eliminate static electricity produced during processes such as rolling paper, wire, and sheet metal.
FAQs
Why is polonium not always classified as a metalloid?
Polonium is sometimes classified as a radioactive element or a post-transition metal because of its high electrical conductivity. Some classifications rather put metalloids under metals.
Is astatine a metalloid?
The formal classification of elements of the periodic table does not include astatine as one of the metalloids. However, recent research has shown that the metal has both metallic and nonmetallic properties, which is typical of a metalloid.
Why don’t metalloids have mechanical applications?
Metalloids do not have mechanical applications because they only share similarities in physical appearance with metals. They behave like nonmetals which is not mechanical. However, metalloids may show different reactions depending on the elements they react with.
Conclusion
The characteristics of metalloids border between the properties of metals and metalloids. However, in these similarities, these characteristics have a way of differentiating metalloids from metals and nonmetals.
Studying the characteristics of metalloids becomes even more difficult with the different classifications of metalloids that have varying numbers of elements.
While there are no clear-cut descriptions for metalloids as a family, you can understand this class of elements if you study them individually.
Also, you should learn about the transition metals – their properties and the reactions they undergo.
Thanks for reading.