Where Are Metalloids on The Periodic Table?

What are the metalloids?

A metalloid is one of the three principal categories of chemical elements, along with metals and nonmetals; metalloids sit neither in the metal or nonmetal categories, but fall somewhere in between with regard to their chemical and physical properties.

The most common examples of metalloids include boron, silicon, germanium, arsenic, antimony, tellurium, and polonium.

If you examine a periodic table of elements (preferably a standard version), you will see that these elements draw a sort of diagonal line across the p-block, beginning with boron, and going down to astatine, where most of them group more towards silicon.

There are even variations of periodic tables that define the difference between metals and nonmetals with an actual line, with metalloids falling just below and to the right of this dividing line.

Most metalloids (again with the exception of polonium) are generally brittle and poor conductors of electricity, although they are less poor conductors than nonmetals.

In their pure form, metalloids get their visual metallic appearance from the metallic luster they often have, although their chemical and physical properties are otherwise more nonmetallic in nature.

While metalloids are more likely to have nonmetal characteristics, they are known to form alloys with metals.

Their physical and chemical properties often alternate or land somewhere between metals and nonmetals as they are associated with both. Due to their brittle structure like common nonmetals metalloids usually are not selected for use as structural elements.

Nevertheless, metalloids and their compounds have been observed and used in practical applications.

You have see them in alloys, catalysts, biological agents, glasses, flame retardants, optical storage, optoelectronic devices, semiconductors, and even fireworks and electronics, among others. Their unique place on the periodic table gives them an incredible amount of versatility.

Where Are Metalloids On The Periodic Table?

Metalloids occupy an intermediate position on the periodic table between metals and nonmetals. The location of the dividing line is a point of contention depending on the version of the periodic table used, often taking a staircase or zigzag pattern.

Generally, the elements found below and left of the dividing line will show more metallic properties, and those above and right of the line will show more nonmetallic properties.

Additionally, it is interesting to note that when a periodic table shows a staircase design, other elements like lithium, beryllium, aluminum, germanium, antimony, and polonium, are often aligned just below the directional line. For whatever reason, these elements are typically the highest critical temperatures in their respective groupings.

The elements with the orange colour in the periodic table are the metalloids, which are positioned between the nonmetals and metals.

So simply put, metalloids reside on the diagonal portion of the p block of the periodic table. Thus, they can also be found to the right of the post transition metals and to the left of the nonmetals.

Characteristic Properties of Metalloids

• While they are called metalloids because of their metallic luster, they are actually more chemically similar to the non-metals.
• For the most part, metalloids exist as brittle solid at room temperature, with a glass-like appearance.
• They have intermediate electrical conductivity that can be, and sometimes is, much closer to that of nonmetals or metals.
• It is their unique electronic band structures that allow them to be classified as semimetals or most often as semiconductors.
• Metalloids additionally have intermediate ionization energies and electronegativities that are not quite low or high.
• Most metalloids are amphoteric or produce oxides that show only slight acidity.
• They can also produce many kinds of municipal alloys.
• In overall conclusion, metalloids are distinctive based on their wide range of intermediate properties, placing them between the nonmetals and metals in the periodic table.

Properties of Metalloids

Metalloids, while often resembling metals, generally behave more like nonmetals. In terms of physical properties, the metalloids are typically brittle solids, with a shiny, metallic luster, and only limited ability to conduct electric current.

Group band structures are especially revealing in showing either semimetals or semiconductors.

In terms of behavior metalloids are usually close to nonmetals. They typically have moderate ionization energies and are in a range of electrically neutral to weakly electronegative. Their oxides are usually amphoteric or at best, weakly acidic.

In general, most of the physical and chemical characteristics of metalloids fall in between those of metals and nonmetals, giving them a middle ground that is one of the unique features of the periodic table.

Physical Properties of Metalloids

The physical properties of metalloids are usually intermediate between metals and nonmetals. There are a few key properties listed below:

• Appearance: Metalloids usually found at room temperature are brittle solids and possess a shiny metallic appearance. Despite their shiny eye-catching luster, metalloids are brittle materials. For example, silicon crystallizes in a blue-grey crystal that is brittle and hard. Antimony appears with a silvery, lustrous gray surface.
• Melting and boiling points: With melting and boiling points, metalloids are generally in between metals and non-metals. Some specific melting points are as follows: silicon melts at 1410 °C, germanium melts at 938.3 °C, and boron melts at 2079 °C. Metalloids have generally lower melting points than metals but higher than non-metals.
• Density: Density is another property where metalloids vary between the respective densities for metals and nonmetals. Density examples include antimony at 6.697 g/cm³, tellurium at 6.24 g/cm³, and arsenic at 5.727 g/cm³.
• Electrical Conductivity: It should be noted that metalloids do not effectively conduct electricity when compared to metals. Some metalloids, such as silicon and germanium, are specifically known for their semiconducting behavior; metalloids can conduct electricity as well as be resistors to ampacity depending on impurity or temperature conditions, etc. This semiconducting functionality not only straddles the differing conductance of electrical energy given by metals and nonmetals but is an essential requirement for conductivity.
• Allotropes: Metalloids are capable of existing in varying physical forms, otherwise known as allotropes, whereby one allotrope may behave differently from the other. One example is arsenic; arsenic is able to exist in gray arsenic, yellow arsenic, and black arsenic, with the gray form being the most common.
• Thermal conductivity: Metalloids are typically better thermal conductors than nonmetals, but do not compare well with true metals. There is a significant range in the capacity to conduct heat with the respective metalloids. In some cases, the increase in thermal conductivity of the elements is utilized in thermoelectric devices.
• Brittleness: A defining characteristic of metalloids is that they are brittle and will likely crack or shatter under stress, not malleable like metals, mentioned earlier. In other words, metalloids do not bend; they break.
• Hardness: Metalloids have a range of hardness. For example, arsenic has a Mohs hardness of 3.5, whereas boron has an impressive density of 9.3; boron is not too far from diamond (10) and significantly harder than gold (2.5).

Chemical Properties of Metalloids

Metalloids generally behave chemically more like nonmetals. Below is a list of some properties of metalloids.  

• Reactivity with Nonmetals: Metalloids have some degree of chemical reactivity, and they tend to form either negatively or positively charged ions by addition or removal of electrons. There is still a unique tendency for metalloid nonmetals to react with nonmetals, forming products, sometimes yielding a wide variety of compounds. For example, silicon can react with halogens to form silicon tetrahalides. Likewise, boron reacts with fluorine to form boron trifluoride.
• Oxidation States: There is a little variation with oxidation states, but it is most often seen in +3, +2, -4, and -2 values. For example, boron is +3 in boron trichloride, whereas silicon is -4 in silicon dioxide. Arsenic and antimony both fall under +3 and +5 oxidation states for most examples.
• Electronegativity: The electronegativity values describe the strength of the attempt for an atom to attract electrons in a bond, and metalloid values typically are in the 1.8 – 2.2 group. The relatively high electronegativity of metalloids allows them to form both covalent and ionic bonds, thus allowing for their relatively versatile chemistry.
• Covalent Bonds: Metalloids tend to form covalent bonds, rather than some metals that form monatomic ions. This factor is due to the metallic and nonmetallic characteristics of other metalloids in the position of the periodic table.
• Alloy Formation: Alloy formation is the other important characteristic. Several metals, like antimony, can form an alloy when combined with another metal. As an example, antimony can be added to lead to form antimonial lead alloys, which are an important working material for manufacturing munitions.
• Reactivity with Acids: Most metalloids will not have a strong reaction or violent reaction to acid exposure. Several metalloids, like silicon, germanium, and polonium, are typically insoluble and form a protective surface oxide when exposed to acids, which prevents further chemical actions. There are metalloids, arsenic, antimony, and bismuth that will undergo some form of oxidation. For metalloids, this oxidation is predominantly when exposed to strong oxidizing acids like nitric acid and not hydrochloric acid, which either does not have the same oxidizing force or a stronger one is needed.

Examples of Metalloids on the Periodic Table

In general, when individuals reference metalloids as they show up on the periodic table, they are probably alluding to boron, silicon, germanium, arsenic, antimony, and tellurium—these elements are most widely accepted as being metalloids.

Nonetheless, there are a handful of elements whose status as metalloids is less universally agreed upon. Carbon, aluminum, selenium, polonium, and astatine are sometimes included in this conversation, but they do not appear on every list.

Commonly Recognised Metalloids

#1. Boron.

Boron is highly versatile and can be incorporated into many different compounds. An example is borosilicate glass, which has an outstanding capacity to resist thermal shock.

Borosilicate glass differs from conventional glass in that it is can withstand sudden swings in temperature without cracking or breaking, making it very useful in laboratories and in kitchens .

Boron filaments also have a unique property of strength with a distinct weight. Because boron filaments are strong and able to be used for reinforcing materials, some aircraft use boron fibers in the airframes to improve strength without adding weight.

You will also find boron fibers incorporated in golf clubs and fishing rods, where strength and lightness are also desirable.

Then we have sodium tetraborate, better known as borax, which should be considered in its applications because there are so many.

Borax is very prevalent as it can be used as a major component of making fiberglass insulation or playing an important role in household detergents and cleaners. Few chemicals can be included in as many consumer products as borax.

#2. Silicon.

Silicon is a prototypical metalloid; it has a luster like metal but is much less malleable and is brittle; a clear indication of its non-metallic character.

It occupies a highly important area, suspended between metals and non-metals, thus explaining its prominence in the electronics industry, particularly in computer chips. Its electrical conductivity falls comfortably in that middle ground for semiconductors.

As a semiconductor, silicon has the interesting property of conducting electricity better at higher temperatures; and nearly 90% of the earth’s crust is silicon compounds (mostly silicates), so you could say silicon is really there.

Of course, silicon is not only found on earth. It is also found throughout the solar system, on moons, in asteroids, and in cosmic dust. Silicates that come from silicon also have an applied use, as the raw ingredient in cement, porcelain, and ceramics.

When you consider the global economy in the 21st century, the impact of silicon cannot be understated. The emergence of semiconductor electronics has transformed industry and daily life.

Importantly, it is not just any form of silicon that is important; pure silicon has played a defining role in the creation of both integrated circuits and transistors. Integrated circuits and transistors are found in nearly every modern electronic device; from your fridge, to your television, or in your pocket (your cell phone).

#3. Germanium.

Germanium has a shiny, gray-white appearance and has density of 5.323 g/cm³. This causes it to be appear distinctly hard, but also brittle, a characteristic all those handling germanium are aware of when first encountering the material.

Germanium is, by nature, unreactive (at room temperature), which means it will be stable under nearly all conditions encountered on a day to day basis, however, hot concentrated sulfuric or nitric acid will be able to oxidize it after a long period of time owing to the corrosive ability of these acid.

However, when molten caustic soda is factor in, germanium reacts in order to create sodium germanate (Na2GeO3) and evolve hydrogen gas upon oxidation. Germanium in its natural elemental state has a melting point of 938 °C and is, therefore, a fairly resilient metal with respect to heat stability when compared to the elements in the periodic table.

The pure elemental forms of germanium is not abundant naturally on Earth, and when found, usually in crystal form, germanium will almost always look like a diamond.

Interestingly, although germanium was only soon isolated as a new element in the periodic table, before it was isolated, Dimitri Mendeleev predicted the existence of germanium, estimated many of its properties, and even organized all of the known elements and those to be discovered in a number of periodic groups. Such insight and scientific clarity demonstrates a substantiative epistemic intuitionism.

In the 21st century, germanium serves a productive use in technology despite being like silicon as a semiconductor in a partial sense. Germanium’s current useful applications are distinct from that of silicon. Today, global strategic development in the advancement of infrared optics, solar energy, and as a component of some metal alloys, have als contributed to a new foundation for germanium.

#4. Arsenic.

Arsenic is a very gray solid that has a metallic luster, with a density of 5.727 g/cm³. While it is somewhat brittle and moderately hard, (it is harder than aluminum but softer than iron), it can be worked with some effort.

When arsenic is in dry air, it does not react. When it is in moisture or water, it will develop a golden-bronze patina that will darken. Arsenic is affected by nitric acid and concentrated sulfuric acid. Arsenic also reacts to fused caustic soda and will yield sodium arsenate (Na₃AsO₃) and hydrogen gas.

Arsenic sublimes at 615 °C, creating a lemon-yellow vapor that has a garlic scent. The metal can melt, but only under high pressure at 38.6 atmospheres and 817 °C.

Arsenic based chemicals readily form covalent bonds with non-metals. This feature is important to a whole host of activities Arsenic is used for, from alloys and electronic components to the historical use of pesticides and herbicides. This has changed explosively in recent years as much of our arsenic use has diminished or been restricted due to human toxicity.

The use of arsenic as an insecticide are also related to its use as a wood preservative. Notable in its toxicity is that arsenic is a group A carcinogen – a cancer risk.

While its toxicity is widely known, arsenic actually has a very low and subtle presence in human biological systems. It seems we need very low levels, perhaps only traces, of arsenic in our body metabolically, but how this actually works is still a research question.

#5. Antimony.

The visually-stunning blue hues of antimony seen in lustrous examples, reveal a silvery-white color. Antimony has a density of 6.697 g/cm³, is brittle, stiff, and relatively hard which helps understand the poor conductivity of antimony.

Antimony has fairly stable characteristics at standard conditions (room temperature and standard air and moisture). Antimony can be seen to improve toughness and hardness when alloyed with lead, valuable for others.

Antimony is of value in the manufacturing of electronic and semiconductor devices, half of the amount of antimony used, is in batteries, projectiles and shots, alloys, solid cabling and comestible features as sink/foundation type works.

As metals and alloys, antimony has a special position as required in semiconductor technology and available a little more pure than common use of metallurgy and percent added to materials. Antimony is very available, to be at about 1/5 the availability of arsenic. Antimony’s atomic context is similar to arsenic’s outermost shell has three half filled shells.

Antimony’s vivele reactivity is comparable to other metalloids but antimony could be considered to bond covalently and bathes very well with halogens. For example, braziers flame burnt in gas burns to the degree that antimony burning produces a blue flame in terms of sulfur reaction.

#6. Tellurium.

Tellurium is a shiny, silver-white solid that weighs 6.24 g/cm³, which distinguishes tellurium as the softest of the metalloids, although harder than sulfur. Tellurium is brittle – even the slightest blow will make it shatter.

As a solid, tellurium is stable in air. However, while in a finely divided powder, tellurium is exposed to air and, under moist conditions, will oxidize.

Tellurium has an interesting chemistry: at boiling water levels, it reacts with water, and in freshly precipitated water, it reacts at low temperatures of 50 °C, forming tellurium dioxide and hydrogen gas as well.

Being a metalloid supports tellurium’s chemistry; tellingly, it reacts easily, especially with compounds with sulfur or selenium. Interestingly, it has a green-blue flame when burned.

From a practical perspective, tellurium is used as an additive in the steel industry, often in alloy with metals such as aluminum, copper, lead, tin, etc. Tellurium will harden ale alloys like antimony, and it makes them less corrodible.

Another interesting reaction of tellurium is its semiconductor properties that become most readily apparent when the element is exposed to light. Tellurium is a rare element in source rock – occurring at lower concentrations in coal deposits and certain plant sources.

Examples of elements That are Irregularly Recognized as A Metalloids

#7. Polonium.

Polonium, symbol Po and atomic number 84, is not prominent in the popular mentality, partly because it is rare to find and extremely radioactive (dangerous). Polonium has no stable isotopes; every isotope of polonium has a half-life that is so brief that you can see why polonium is not easily found.

While polonium is often classified as a metal and occasionally as a metalloid, polonium is located as a chalcogen alongside selenium and tellurium on the periodic table.

Polonium has some similar chemical behavior to selenium and tellurium, but its metallic properties associate it more closely with the other metals in the group, thallium, lead, and bismuth, so polonium occupies something of an intermediate position.

Since all isotopes of polonium are so short-lived comparatively, it has little time to exist in nature. The only significant examples you are likely to see are polonium 210 (210Po) in uranium ores, and the polonium itself does not last very long since its half-life is about 138 days and it appears in the final decay series of uranium-238 (238U).

#8. Astatine.

With ‘At’ as its chemical symbol, Astatine occupies the 85th atomic number on the periodic table. Being a halogen, it is the heaviest member of the group, as well as sharing some of its chemical properties with iodine.

Radioactivity is the most significant property of astatine. The isotopes of astatine, according to researchers, have an average half life of about 8.1 hours, although some decay even shorter time spans.

Approximately seven isotopes are known, the majority of which are unstable. When found, astatine is generally black with a minor metallic luster.

The true distinct feature of astatine is its limited supply. It is one of the least abundant naturally occurring elements on Earth. To give a sense of rarity, the entire crust of Earth is made up of 2.36 × 10²⁵ grams of astatine, yet less than one gram is astatine.

Most astatine found in nature is as a byproduct of the decay chains of heavier elements such as thorium or uranium.

Examples of elements That are Less Commonly recognized as a Metalloids

#9. Selenium.

Selenium, represented by the symbol Se and having the atomic number 34, may appear in several different physical forms.

Some forms you may encounter are a powder that is red brick in color, a metallic solid that is shiny grey in color, and a black, glassy, vitreous form which is much different from the powder and solid forms.

As strange as it seems, following the appearance of Selenium you may see that it is rarely found in its pure form or in the occurrence of primary mineral deposits in the crust of the Earth. Selenium was first identified in 1817 by the chemist Jöns Jacob Berzelius.

In fact, when Berzelius first saw Selenium, he described it as being so similar to tellurium that selenium gets its name from tellurium, which was discovered not long before it.

Selenium is usually found in nature in the metal sulfide ores, where it has replaced sulfur. Commercially, selenium is recovered as a by-product during the refining of metal sulfide ores that have appreciable concentrations of base metals.

There is very little pure selenide or selenate minerals (commonly referred to as “selenium) and as a result, most selenium is not mined and obtained from its dedicated ore. Currently, commercial features the use of selenium in glasses and pigments.

Historically selenium played an important role in electronics, including as a semiconductor and in the manufacture of photocells, but with advancements in technology, silicon has now replaced selenium for the same functions.

However, selenium can still be found in various applications including in certain electronic devices (e.g., DC power surge protectors) and fluorescent quantum dots.

Examples of elements That are Rarely recognized as a Metalloids

#10. Carbon.

Carbon is denoted by the letter C and has an atomic number of 6. In total, it is approximately 0.025 percent of the Earth’s crust. It may seem like a small amount but it is nevertheless the 15th most abundant element in the crust.

More interestingly, if one looked at the universe as a whole, carbon would be the 4th most abundant element in our cosmos behind hydrogen, helium and oxygen.

One of the unique features of carbon is how much variety it has in forming compounds. This structural variety, coupled with the fact that carbon rarely forms polymers at Earth’s temperatures is the reason carbon can be the backbone of all known life. Indeed, carbon ranks 2nd to oxygen in human mass, making up 18.5% of human mass.

#11. Aluminum.

Aluminum is the most common metal in the Earth’s crust, being about 8.1% of the crust’s total composition. It is interesting to note that although aluminum is the most common metal, you will rarely find the pure metal taking form in nature.

In fact, it is typically found in the form of minerals that contain aluminum and silica, such as bauxite and cryolite (which you will find referenced quite often in regard to industrial aluminum production).

Bauxite and cryolite are both considered aluminum silicates, and they are the main sources for producing aluminum commercially, particularly through the Hall-Héroult process.

So why is there no metallic aluminum in nature? Because aluminum has a very high degree of chemical reactivity, so aluminum ends up forming compounds instead. In fact, aluminum compounds are usually found in pretty much every stone, plant, and animal.

Of the outer 16 kilometers (or 10 miles) of the crust, aluminum accounts for about 8% of its weight, so it is also abundant in the Earth’s crust, just behind oxygen and silicon in contribution.

In regard to the name “aluminum” (or “aluminium” depending on where you are from) means “bitter salt.” The name comes from the Latin word “alumen,” which means potash alum (aluminum potassium sulfate) __ KAl(SO₄)₂·12H₂O.

Applications of Metalloids

Metalloids usually have a metallic luster, but they tend to be very brittle, and their conductivity tends to be less than true metals. What they have in common is that their physical and chemical properties typically tend to fall between metals and nonmetals.

What is interesting about metalloids is that they can also form alloys with many metals. This interoperability indicates how many useful applications exist for both the metalloids and compounds that they form.

Metalloids, or the minerals they form, have uses in alloys, biological applications, catalyst applications, flame retardants, types of glass, optical storage, and optoelectronic applications.

Also, metalloids and their derivatives have been identified on purpose in electronics, semiconductors, or even pyrotechnics. Very light metalloids, such as boron, would be the featured element if we consider alloys with transition metals.

In this case, boron is quite known for forming intermetallic compounds. Boron can form combinations of metals and these compounds can be expressed as MnB when n is greater than 2.

For example, ferroboron has ~15% boron and is often used to introduce boron in steel. There are also nickel-boron alloys for welding materials or case hardening metals in the engineering field.

Silicon alloys with aluminum and iron have become common metals in construction and automotive industries. Germanium can form multiple different alloys, especially with the coinage metals.

Medical Applications of Metalloids

Altogether there are six metalloids, and they occupy a very unique position in science, sitting in-between toxic and non-toxic with some of them being known toxins (by this I mean Antimony and Arsenic) and others are trace elements vital for biological functioning (stimulated by Boron, Arsenic and Silicon nutrition and physiology as an animal).

Boron, Arsenic, Silicon, and Antimony all have their various uses in the medical realm ranging from diagnostics to therapeutics.

The other two, Germanium and Tellurium, do not yet have a use, but there are ongoing research studies that show promising applications in medical science.

As it pertains to location of boron, a lot of boron appears in contexts one may not expect. Boron isn’t just a trace element, but also commonly is utilized in herbicides and insecticides.

Furthermore, of the boron compounds, boric acid has antiseptic, antiviral, and antifungal characteristics, allowing it utility in both health care and agriculture.

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