What are the metalloids?
A metalloid is an element that has properties that are intermediate between those of metals and nonmetals. Metalloids can also be called semimetals. On the periodic table, the elements colored green, which generally border the stair-step line, are considered to be metalloids.
Notice that aluminum borders the line, but it is considered to be metal since all of its properties are like those of metals.
There is no standard definition of a metalloid and no complete agreement on which elements are metalloids. Despite the lack of specificity, the term remains in use in the literature of chemistry.
The six commonly recognized metalloids are boron, silicon, germanium, arsenic, antimony, and tellurium. Five elements are less frequently so classified: carbon, aluminum, selenium, polonium, and astatine.
On a standard periodic table, all eleven elements are in a diagonal region of the p-block extending from boron at the upper left to astatine at the lower right. Some periodic tables include a dividing line between metals and nonmetals, and the metalloids may be found close to this line.
Some allotropes of elements show more pronounced metal, metalloid, or non-metal behavior than others. The element carbon; its diamond allotrope is non-metallic; however, the graphite allotrope is electrically conductive showing characteristics more like a metalloid. Phosphorus, tin, selenium, and bismuth also have allotropes that display borderline behavior.
Metalloids tend to have a metallic appearance, but behave more like nonmetals in most chemical reactions. All Metalloids are solid at room temperature. They are much more brittle than metals but are much poorer electrical conductors. The hybrid characteristics metalloids possess offer a broad range of real-world applications such as metal alloys, flame retardants, and semiconductors /electronics.
Where Are Metalloids On The Periodic Table?
Metalloids lie on either side of the dividing line between metals and nonmetals. This can be found, in varying configurations, on some periodic tables. Elements to the lower left of the line generally display increasing metallic behavior; elements to the upper right display increasing nonmetallic behavior. When presented as a regular stairstep, elements with the highest critical temperature for their groups (Li, Be, Al, Ge, Sb, Po) lie just below the line.
Metalloids are located between metals and nonmetals. The orange color on the Periodic table represents metalloids. They form a separating boundary between metals and nonmetals.
In other words, metalloids (semimetals) are located on the right side of the post-transition metals and on the left side of nonmetals. Also, we can say that metalloids are present in the diagonal region of the p block on the Periodic table.
Common Properties of Metalloids
Metalloids have properties that are between the properties of nonmetals and metals. Most metalloids have:
- An appearance that is similar to metals.
- They are less conductive than metal.
- They are more brittle than metals.
- Metalloids have nonmetallic chemical properties in general.
- The electronegativities of metalloids are between those of nonmetals and metals.
- The ionization energies of metalloids are also between those of nonmetals and metals.
- Semimetals/metalloids have some characteristics of nonmetals and some characteristics of metals.
- The reactivity of metalloids depends on the properties of the elements they are interacting with.
- Metalloids tend to be good semiconductors.
- Metalloids may have a metallic luster, but they also have tropes that can have a nonmetallic appearance.
- Metalloids are usually brittle, and they are also typically solid, only becoming non-solid under uncommon conditions.
- Metalloids typically behave as nonmetals in chemical reactions, and they can create alloys with metals.
Summing up, let’s take a quick look at both the physical properties and chemical properties of metalloids.
Physical Properties of Metalloids
Physical properties are characteristics that can be documented or observed without altering the substance of the element, without changing the group of molecules into substances. Physical properties include things like the freezing point and density.
The physical properties of metalloids are as follows:
- Metalloids have a solid state of matter.
- In general, metalloids have a metallic luster. Metalloids have low elasticity; they are very brittle.
- Middleweights are semi-conducted elements, and they allow leave the average transmission of heat.
Chemical Properties of Metalloids
Chemical properties are those which defined how a substance interacts/reacts with other substances or changes from one substance to another substance. Chemical reactions are the only time that the chemical properties of an element can be quantified. Chemical reactions include things like rushing, burning, tarnishing, exploding, etc.
The chemical properties of metalloids are as follows:
- Metalloids easily form gasses when they oxidize.
- Metalloids can be combined with metals to create alloys.
- Metalloids have different metallic and non-metallic allotropes.
- When metalloids melt some of them will contract.
- Metalloids can react with halogens to form compounds.
Characteristic Properties of Metalloids
- Metalloids are solids
- They have a metallic luster, and generally, look like metals
- They are brittle, and easily shattered
- Metalloids can conduct electricity, but not as well as metals.
- Chemically, they act more like nonmetals, easily forming anions, having multiple oxidation states, and forming covalent bonds.
- Their ionization energies and electronegativities are in between the values of metals and nonmetals.
Metalloids are by far the smallest group of elements, as there are only six elements definitively classified as metalloids. They can have anywhere from three to six valence electrons in their outer energy shell. This is the driver of their reactivity/chemical behavior.
Boron, which only has three valence electrons, behaves much like metal during chemical reactions by giving up its electrons. The other metalloids, with four or more valence electrons, tend to behave more like nonmetals, gaining electrons during reactions.
Let’s learn some facts about the individual metalloids, starting with boron.
Examples of Metalloids on the Periodic Table
Boron is a versatile element that can be incorporated into a number of compounds. Borosilicate glass is extremely resistant to thermal shock. Extreme changes in the temperature of objects containing borosilicate will not create any damage to the material, unlike other glass compositions, which would crack or shatter.
Because of their strength, boron filaments are used as light, high-strength materials for airplanes, golf clubs, and fishing rods. Sodium tetraborate is widely used in fiberglass as insulation and also is employed in many detergents and cleaners.
Silicon is a typical metalloid. It has luster like a metal but is brittle like a nonmetal. Silicon is used extensively in computer chips and other electronics because its electrical conductivity is in between that of a metal and a nonmetal.
It is a potent semiconductor, meaning it conducts electricity more efficiently at higher temperatures. Silicon compounds called silicates make up almost 90% of the earth’s crust, pure silicon is rare. It is, however, relatively common in asteroids, moons, and cosmic dust. Silicates are frequently used in the manufacturing of cement, porcelain, and ceramics.
In the 21st century, silicon has had a massive influence on the world economy through its importance in the development of semiconductor electronics. Pure silicon has been vital to the development of integrated circuit chips and transistors, both of which are crucial components of modern electronic devices, such as cell phones, televisions, and household appliances.
Germanium is a shiny grey-white solid. It has a density of 5.323 g/cm3 and is hard and brittle. It is mostly unreactive at room temperature but is slowly attacked by hot concentrated sulfuric or nitric acid. Germanium also reacts with molten caustic soda to yield sodium germanate Na2GeO3 and hydrogen gas. It melts at 938 °C.
It is also a good semiconductor and is rarely found in its pure elemental form on earth. Germanium frequently crystallizes into a diamond structure. Germanium was predicted to exist by Dimitri Mendeleev years before it was actually discovered. He was also able to predict many of its properties using his understanding of periodic trends and knowledge of other metalloids and nearby elements.
Like silicon, germanium is also critical to modern technology, although it is primarily used in different applications than its metalloid cousin. Germanium is often used for infrared optics, solar energy, and numerous metal alloys.
Arsenic is a grey, metallic-looking solid. It has a density of 5.727 g/cm3 and is brittle, and moderately hard (more than aluminum; less than iron). It is stable in dry air but develops a golden bronze patina in moist air, which blackens on further exposure.
Arsenic is attacked by nitric acid and concentrated sulfuric acid. It reacts with fused caustic soda to give the arsenate Na3AsO3 and hydrogen gas. Arsenic sublimes at 615 °C. The vapor is lemon-yellow and smells like garlic. Arsenic only melts under a pressure of 38.6 atm, at 817 °C.
It readily forms covalent bonds with nonmetals. Arsenic has applications with regard to alloys, electronics, and pesticides/herbicides, but the use of arsenic for these applications is decreasing due to the toxicity of the metal.
Its effectiveness as an insecticide has led arsenic to be used as a wood preservative. It is classified as a Group-A carcinogen. Despite its toxicity, very small quantities of arsenic are required for human metabolism, but the mechanism for this is unknown.
Antimony is a silver-white solid with a blue tint and a brilliant luster. It has a density of 6.697 g/cm3 and is a brittle and moderately hard material that is a poor conductor of electricity. It is stable in air and moisture at room temperature. Used with lead, antimony increases the hardness and strength of the mixture. This material plays an important role in the fabrication of electronic and semiconductor devices.
About half of the antimony used industrially is employed in the production of batteries, bullets, alloys, cables, and plumbing equipment. As is consistent with other metalloids, highly purified antimony can be used in semiconductor technologies.
It is found in nature at about ⅕ the abundance of Arsenic. Antimony has a similar atomic structure to arsenic as well, with three half-filled electron shells in the outermost shell. It typically forms covalent bonds and is highly reactive with halogens, such as sulfur, and produces a brilliant blue flame when burned.
Tellurium is a silvery-white shiny solid. It has a density of 6.24 g/cm3, is brittle, and is the softest of the commonly recognized metalloids, being marginally harder than sulfur. Large pieces of tellurium are stable in the air. The finely powdered form is oxidized by air in the presence of moisture. Tellurium reacts with boiling water, or when freshly precipitated even at 50 °C, to give the dioxide and hydrogen.
Tellurium is a metalloid that exhibits a similar description to antimony. Tellurium is highly reactive with sulfur and selenium and shows a green-blue flame when burned. Tellurium is industrially used as a steel additive and can be alloyed with aluminum, copper, lead, or tin.
Like antimony, tellurium can also strengthen other metals, but can also reduce corrosion when added to the aforementioned metals. Additionally, tellurium serves as a strong semiconductor, particularly when exposed to light. In nature, most tellurium is found in coal, though trace amounts are found in some plants.
Polonium is “distinctly metallic” in some ways. Both of its allotropic forms are metallic conductors. It is soluble in acids, forming the rose-colored Po2+ cation and displacing hydrogen: Po + 2 H+ → Po2+ + H2. Many polonium salts are known. The oxide PoO2 is predominantly basic in nature.
Polonium is a reluctant oxidizing agent, unlike its lightest congener oxygen: highly reducing conditions are required for the formation of the Po2− anion in an aqueous solution.
Whether polonium is ductile or brittle is unclear. It is predicted to be ductile based on its calculated elastic constants. It has a simple cubic crystalline structure. Such a structure has few slip systems and “leads to very low ductility and hence low fracture resistance”.
Polonium shows a nonmetallic character in its halides, and by the existence of polonides. The halides have properties generally characteristic of nonmetal halides. Many metal polonides, obtained by heating the elements together at 500–1,000 °C, and containing the Po2− anion, are also known.
As a halogen, astatine tends to be classified as a nonmetal. It has some marked metallic properties and is sometimes instead classified as either a metalloid or (less often) as a metal. Immediately following its production in 1940, early investigators considered it a metal.
In 1949 it was called the most noble (difficult to reduce) nonmetal as well as being a relatively noble (difficult to oxidize) metal. In 1950 astatine was described as a halogen and (therefore) a reactive nonmetal. In 2013, on the basis of relativistic modeling, astatine was predicted to be a monatomic metal, with a face-center cubic crystalline structure.
Several authors have commented on the metallic nature of some of the properties of astatine. Since iodine is a semiconductor in the direction of its planes, and since the halogens become more metallic with increasing atomic number, it has been presumed that astatine would be metal if it could form a condensed phase.
Astatine may be metallic in the liquid state on the basis that elements with an enthalpy of vaporization greater than ~42 kJ/mol are metallic when liquid. Such elements include boron, silicon, germanium, antimony, selenium, and tellurium.