Metalloids: metalloids periodic table facts

What are metalloids? a plain definition

A metalloid is a chemical element whose properties are a mix of those of metals and nonmetals, or fall somewhere in between, so that it is hard to file under either heading. The word comes from the Latin metallum (metal) and the Greek oeidēs (resembling in form), so a metalloid is literally "metal-like." That description fits: most metalloids have a grey, shiny surface, yet they shatter when struck and carry electricity far less freely than copper or iron. For anyone interested in metallurgy, pages on vanadium facts connect the chemical element to steel, alloy strength and mining of ore.

Like every other element, a metalloid is made of atoms with protons and neutrons in the nucleus and electrons around it. Nothing about their make-up is exotic, and in basic chemistry they are treated like any other elements. What makes them a separate group is behaviour: their outer electrons are held more tightly than a metal's but less tightly than a typical nonmetal's, so their chemistry lands in the middle.

The key point for you is that there is no single official definition of the group, and so no complete agreement on which elements belong to it. The classification is a convenience that chemistry teachers and materials scientists use, not a rule set by a standards body.

Judgment-based vs criteria-based metalloid definitions

Authors who write about metalloids tend to use one of two approaches.

Judgment-based

An element is a metalloid if it has a mixture of metallic and nonmetallic properties and lacks a clear majority of either, which makes it hard to classify. Most elements show some of both traits; only those right at the margin, with no clear winner, are treated as metalloids.

Criteria-based

An element is a metalloid if it meets one or more measurable criteria, such as an electronegativity in a set band or a semiconducting band structure.

The measurable criteria most often cited are:

• Electronegativity of roughly 1.8 or 1.9 up to 2.2 on the revised Pauling scale.
• A first ionization energy in an intermediate band, often quoted as about 750 to 1,000 kJ/mol.
• An electronic band structure of a semiconductor or a semimetal in the element's standard state.
• Packing efficiency (the fraction of a crystal's volume filled by atoms) of about 34% to 41%, well below the 68% to 74% typical of metals.
• A Goldhammer–Herzfeld ratio near 1, a simple measure of how metallic an element is.

The Goldhammer–Herzfeld ratio compares the size of an atom with the space it occupies in the bulk solid:

Here \(R\) is the atomic radius and \(V_{m}\) the molar volume, with constants folded in. Values well above 1 point to metals, values well below 1 point to nonmetals, and the recognised metalloids cluster around 0.85 to 1.1.

The trouble with any single criterion is that it draws a slightly different border each time. Lists in the literature range from four elements to more than a dozen, and on average a list names about seven.

Why metalloids are also called semimetals

You will often see metalloids called semimetals or semi-metals, especially in older textbooks and introductory courses. Chemists now discourage that use, because in physics a semimetal is something specific: a material whose conduction and valence bands overlap very slightly, giving it a small number of charge carriers without a band gap. By that physics definition only arsenic and antimony among the commonly recognised metalloids are true semimetals, while bismuth and graphite also qualify even though they are not usually listed as metalloids.

The name itself has a history. In the early 1800s "metalloid" was used for nonmetals in general, and the modern meaning, a category of elements with intermediate or hybrid properties, only became widespread between about 1940 and 1960. If you read an old chemistry text that calls sulfur or phosphorus a metalloid, that is the older usage, not a mistake.

Where the metalloids sit on the periodic table

On a standard periodic table the metalloids form a diagonal band in the p-block, running from boron in group 13 at the upper left down through silicon and germanium in group 14, arsenic and antimony in group 15 and tellurium in group 16. The less commonly counted candidates continue the same band to astatine at the lower right. Their atomic numbers jump around (boron is 5, tellurium is 52) because the band cuts diagonally across periods rather than running down one column of the periodic table, which is why these elements are easy to miss if you scan by group. Before choosing a surgical implant material, read the titanium atomic mass page on the passivation layer and corrosion resistance.

The stair-step line between metals and nonmetals

Many periodic tables draw a stair-step line, also called the dividing line, between boron and aluminium, then down between silicon and aluminium, germanium and arsenic, antimony and tellurium, and polonium and astatine. Elements to the lower left of the dividing line become steadily more metallic; elements to the upper right become more nonmetallic. Metalloids sit on either side of this staircase.

The line is a teaching aid, not a physical boundary. Aluminium touches it yet is a metal by almost every test, and some tables leave the line out entirely, showing the metalloids as a shaded diagonal band or a fuzzy transition region instead.

If you see two tables that disagree on where to draw it, both are using a convention, and neither is wrong. What stays constant is the gradient in metallic character across the band.

The diagonal relationship behind the metalloid band

Elements with similar properties usually sit in the same vertical group, so a diagonal band of look-alikes is unusual. It comes from two competing periodic trends.

Moving across a period, nuclear charge rises faster than the shielding from added electrons, so atoms shrink, ionization energy climbs and the elements turn from strongly metallic to strongly nonmetallic. Moving down a group, the outer electrons sit farther from the nucleus, atoms grow, ionization energy falls and metallic character rises.

Put those together and an element one step down and one step right ends up with roughly the same balance as its neighbour. This diagonal relationship is why boron resembles silicon more than it resembles aluminium directly beneath it, and why the transition zone between metals and nonmetals drifts to the right as you go down the periodic table.

Alternative classification schemes

Not every author uses a separate metalloid category at all. Some keep a strict two-way split so that rules for predicting bond types stay simple; in those schemes the border elements are grouped with the metals, with the nonmetals, or treated as a nonmetallic sub-category.

Others argue that keeping a metalloid group is useful precisely because it shows that the properties of elements change gradually across the periodic table rather than switching abruptly. If you are studying chemistry for a course, follow the scheme your textbook uses and note which elements it counts.

List of metalloids: the six commonly recognised metalloids

Six elements appear on almost every list. The table below collects their basic data; the values are standard published constants.

Notice how tightly the electronegativity values bunch together, between 1.90 and 2.18, even though the six metalloids span four groups and four periods. That narrow band is the clearest numerical sign that these elements belong together. Here is a closer look at each, with the examples of metalloids you are most likely to meet in daily life.

Boron

Boron is the lightest metalloid and the only one in group 13. Pure boron is a hard, dark solid, and crystalline boron is almost as hard as diamond. In nature it never occurs free; it turns up as borax and other borates in dry lake beds and as boric acid in volcanic regions.

Boron oxide is a key ingredient in borosilicate glass, the heat-resistant glass used in laboratory beakers and ovenware, because it lowers the glass's thermal expansion so it does not crack under sudden temperature changes. Boron fibres reinforce lightweight composites, and boron carbide is one of the hardest materials known, used in armour plating and cutting tools.

Silicon

Silicon is the best-known metalloid and the second most abundant element in the earth's crust after oxygen. You meet it every day as silica (silicon dioxide) in sand and quartz, and in the silicate minerals that make up most rocks, soils and clays. It has a blue-grey metallic luster but breaks like glass.

Purified silicon is the base of nearly all computer chips and solar cells, because its electrical conductivity sits between that of a metal and an insulator and can be tuned very precisely. Silicon also strengthens aluminium casting alloys and forms the backbone of silicone polymers.

Germanium

Germanium sits directly below silicon and behaves much like it. It was the material of the first practical transistors in the late 1940s, before silicon took over because it handles heat better.

Today germanium is valued for its optical behaviour: it is transparent to infrared light, so it is used in thermal-imaging lenses, and germanium dioxide is added to the glass core of fiber optics to raise its refractive index. Germanium also appears in high-efficiency solar cells for satellites.

Arsenic

Arsenic is famous as a poison, and its compounds are highly toxic, but it has real technical uses. The most stable form, grey arsenic, is a brittle, metallic-looking semimetal. Combined with gallium it forms gallium arsenide, a semiconductor used in LEDs, laser diodes and high-frequency chips in phones.

Arsenic compounds were once common in pesticides and as a wood preservative (chromated copper arsenate), though both uses have declined sharply because of the health risks. Small additions of arsenic also harden lead alloys.

Antimony

Antimony is a brittle, bluish-white metalloid and, like arsenic, a semimetal in physics terms. Its biggest use is as antimony trioxide in flame retardants for plastics, textiles and cable insulation.

Added to lead, antimony makes the alloy harder and more resistant to corrosion, which is why it is used in the grids of lead-acid batteries and in some bullets and solders. Antimony compounds also serve as catalysts in making polyester.

Tellurium

Tellurium is a silvery-white, brittle metalloid and one of the rarest stable elements in the earth's crust. Most of it is recovered as a by-product of copper refining.

Tellurium's main markets are cadmium telluride thin-film solar panels and thermoelectric devices made from bismuth telluride, which turn heat directly into electricity (or electricity into cooling). Small amounts improve the machinability of steel and copper alloys.

Elements less commonly recognised as metalloids

Five more elements are added to the list by some authors and left off by others: carbon, aluminium, selenium, polonium and astatine. Each has at least one metalloid trait, but each also leans clearly towards metal or nonmetal by other measures, which is why the call is contested. Each is a borderline metalloid element at best.

Carbon and aluminium

Carbon is normally treated as a nonmetal, yet its graphite form is lustrous, has a high electrical conductivity along its layers and is a semimetal in band terms, which is why a few sources count it. Diamond, the other main allotrope of carbon, is a transparent insulator, so overall carbon fits better with the nonmetals. Carbon's chemistry, with its endless chains of carbon–carbon bonds, is also quite unlike that of silicon.

Aluminium (aluminum in American spelling) sits right on the stair-step line and forms an amphoteric oxide, which reacts with both acids and bases, a typical metalloid trait. Some of its compounds also behave in a covalent, nonmetal-like way.

But aluminium is malleable, ductile and an excellent conductor, so almost every chemist classifies it as a metal. Carbon and aluminium are therefore rarely counted, appearing on only about one list in ten.

Selenium, polonium and astatine

Selenium sits in group 16 between sulfur and tellurium. Its red, molecular form is an insulator, which is why most chemists call selenium a nonmetal, but grey selenium is a photoconductor: its electrical conductivity rises sharply in light, which made selenium the material of early photocells and photocopier drums. That semiconducting behaviour earns selenium a place on roughly a quarter of metalloid lists, and selenium compounds still tint and decolourise glass.

Polonium, below tellurium, is intensely radioactive and metallic in most respects, though its oxide is amphoteric. Astatine, the heaviest naturally occurring halogen, is so radioactive that no visible amount of it has ever been collected; predictions of its properties range from metalloid to metal.

Because polonium and astatine are so hard to study, their placement rests partly on calculation rather than measurement. Bismuth, the heavy group 15 metal next to polonium, is sometimes mentioned alongside them because it is a semimetal like antimony.

Near metalloids and superheavy elements

A wider ring of elements is very occasionally called metalloid: hydrogen, beryllium, phosphorus, sulfur, gallium, tin, iodine, lead and bismuth, for example. Most of these have one striking intermediate trait, such as tin's grey, semiconducting low-temperature form or iodine's metallic sheen.

The superheavy elements at the bottom of the p-block (flerovium through tennessine) are sometimes predicted to be metalloids too, but they last only seconds or less, and relativistic effects on their electrons make their chemistry hard to predict. For everyday purposes you can treat these as metals or nonmetals that happen to share a metalloid feature.

Physical properties of metalloids

The physical properties of metalloids follow a consistent pattern: they look like metals but handle like nonmetals. All six commonly recognised metalloids are solid at room temperature, they have a metallic appearance, and they are typically brittle. Their densities are moderate, lower than most structural metals, which is part of what makes boron and silicon attractive in lightweight parts.

Metallic luster but brittle

Freshly broken silicon, germanium, antimony and tellurium are all lustrous, with the shiny grey metallic luster you would expect from a metal. That is usually the quickest way to spot a metalloid by eye, since its chemical properties are much harder to read at a glance. But tap one with a hammer and it shatters instead of denting.

Metalloids are brittle, not malleable or ductile: their atoms are held in place by directional covalent bonds, so layers cannot slide past one another the way they do in a metal. When stressed, a metalloid cracks along planes or crumbles. In practice this means you cannot roll, draw or cold-form a metalloid the way you would steel or copper, and you will almost never see one used as a structural part on its own.

Melting points of the metalloid elements

The metalloids have relatively high melting points, though they spread across a wide range. Boron's is among the highest of any element, while tellurium melts at a temperature you could reach on a kitchen stove's hot plate.

Arsenic is the odd one out: at normal pressure it skips the liquid state and turns straight to vapour, so its "melting point" only exists in a sealed, pressurised container.

Crystal structure and allotropes

Covalent bonding sets the crystal structure of every metalloid, which is another way they resemble nonmetals. Silicon and germanium crystallise in the diamond structure: each atom bonds to four neighbours at the corners of a regular tetrahedral arrangement, so a single crystal is effectively one giant molecule. Grey arsenic and antimony form puckered layers in which each atom bonds to three others, a little like a crumpled sheet of graphite.

Tellurium builds endless spiral chains, each atom linked to two neighbours. Boron is the most complex of all: its crystals are built from 12-atom icosahedra, twenty-faced clusters packed together in several different ways.

Many metalloids have more than one form, known as allotropes. Arsenic has a yellow, molecular form made of As₄ units as well as its stable grey form; antimony has explosive and amorphous forms; boron exists in several crystalline and amorphous forms. Which allotrope you are holding can change the properties a great deal, which is one more reason metalloid classification is so slippery.

Chemical properties of metalloid elements

Chemically, metalloids mostly behave like weak nonmetals. They have intermediate ionization energies and electronegativity values, and they form oxides that are amphoteric or weakly acidic. In chemical reactions with very electronegative nonmetals such as fluorine or oxygen they can give up electrons and behave a little like metals, though they rarely form simple cations in solution the way sodium or calcium do.

With active metals they can take electrons, forming compounds such as arsenides and tellurides. That two-way behaviour, more than any single number, is what metalloid properties look like in the lab.

Covalent bonds and oxidation states

Metalloids prefer to share electrons rather than transfer them, so most of their compounds are held together by covalent bonds. Their electron configuration explains the patterns: boron has three valence electrons (2s² 2p¹), silicon and germanium four, arsenic and antimony five, and tellurium six. They show several oxidation states each:

• Boron: +3 in almost all stable compounds, with unusual fractional states in boron hydrides.
• Silicon and germanium: +4 most often, with +2 also known for germanium.
• Arsenic and antimony: +3 and +5, and −3 in arsenides and antimonides, where they form anions.
• Tellurium: +4 and +6 with oxygen and halogens, −2 in tellurides.

Boron is a special case: with only three valence electrons for four available orbitals it is electron-deficient, so it forms unusual bonds where three atoms share a single pair of electrons.

Ionization energy and electronegativity

Ionization energy is the energy needed to strip the outermost electron from a neutral atom; electronegativity measures how strongly an atom pulls on shared electrons in a bond. For metals both are low, which is why metals lose electrons easily and form positive ions.

For nonmetals like fluorine and oxygen both are high. The metalloids land in the middle, with first ionization energies of about 760 to 950 kJ/mol and electronegativities of about 1.9 to 2.2.

A useful comparison: sodium, a typical metal, has an electronegativity of 0.93 and chlorine, a typical nonmetal, 3.16. Silicon's 1.90 sits almost exactly halfway. When a metalloid bonds with an element of similar electronegativity, the bond is close to pure covalent; when it bonds with something far more electronegative, such as fluorine, the bond becomes polar and the metalloid carries a partial positive charge.

Amphoteric oxides, halides and Lewis acids

The oxides of the metalloids show their in-between nature clearly. Metal oxides are basic and nonmetal oxides are acidic, while metalloid oxides tend to be amphoteric (able to react with both acids and bases) or weakly acidic. Germanium, arsenic(III), antimony(III) and tellurium oxides are amphoteric; boron oxide and silicon dioxide are weakly acidic.

Silicon dioxide is the most important example. It is inert to almost every acid except hydrofluoric acid, which attacks it because the silicon–fluorine bond is even stronger than the silicon–oxygen bond.

That is why you can store nearly every common acid in a glass bottle, but never HF. Hot concentrated bases, on the other hand, slowly dissolve silica, a classic sign of acidic oxide behaviour.

The halides are just as telling. Boron trifluoride (BF₃) and silicon tetrafluoride (SiF₄) are both Lewis acids: boron's empty p orbital and silicon's ability to expand its bonding let them accept an electron pair from a donor. Most boron and silicon halides react quickly with water, giving boric acid or silicic acid plus the matching hydrohalic acid, which shows how readily these compounds react with acids and bases alike.

Metalloids as semiconductors: electrical and thermal conductivity

The single most useful property of metalloids is that several of them are semiconductors. Their electrical conductivity is far lower than a metal's but far higher than an insulator's, and, unlike a metal, it rises as temperature increases. Pure silicon at room temperature conducts many millions of times less well than copper, yet it is still a far better conductor than glass or rubber.

That middle position, an electrical conductivity that can be raised or lowered on demand, is what lets engineers switch it between acting like conductors and acting like insulators. Thermal conductivity is moderate to good: silicon moves heat well enough (about 150 W/m·K) to work as a heat-spreading substrate, which matters in power electronics.

Band structure: semiconductor or semimetal

Whether a metalloid conducts depends on its band structure, the energy levels available to its electrons. In a metal the highest filled band overlaps an empty one, so electrons move freely. In an insulator a wide gap separates the filled valence band from the empty conduction band.

A semiconductor has a narrow gap that heat or light can bridge: about 1.1 electron-volts for silicon, 0.67 eV for germanium and roughly 0.33 eV for tellurium. Boron is also a semiconductor, with a wider gap.

Arsenic and antimony are different. Their bands overlap very slightly, which makes each a semimetal rather than a semiconductor: their electrical conductivity is reasonably good, closer to a poor metal, but carry far fewer charge carriers than a true metal. So when a source says "metalloids are semiconductors," it is right for four of the six and only approximately right for the other two.

Doping silicon and germanium

Pure silicon and germanium are poor conductors on their own. Their usefulness comes from doping: adding tiny, controlled amounts of an impurity, sometimes one atom in many millions, to change how they carry charge. Adding phosphorus or arsenic (five valence electrons) supplies extra electrons and makes n-type material.

Adding boron (three valence electrons) creates "holes" and makes p-type material. Joining n-type and p-type regions gives the diodes and transistors inside microchips and the junctions that turn light into current in solar cells. It is a neat detail that three metalloids, silicon, boron and arsenic, often end up in the same chip.

Metalloids vs metals and nonmetals

The quickest way to understand metalloids is to line them up against the two groups they sit between. The table below compares typical metals and nonmetals with the metalloids across the properties chemists use most.

Read across any row and the pattern is the same: on physical properties the metalloids side with metals on looks, and with nonmetals on brittleness; on chemical behaviour they lean nonmetallic. In short, metals are dense, conductive and workable, nonmetals are mostly light, insulating and either gaseous or brittle, and metalloids are shiny, brittle semiconductors that react like weak nonmetals.

Uses of metalloids in industry and technology

Metalloids and their compounds turn up in alloys, biological agents, catalysts, flame retardants, glasses, optical storage, pyrotechnics and, above all, electronics. Silicon alone underpins the modern technology industry, but each of these elements has at least one job where its intermediate nature is the whole point.

Alloys and flame retardants

Metalloids are rarely used as structural materials themselves, but small additions of these elements change metals a great deal. Silicon is the main alloying addition in aluminium casting alloys, improving how the molten metal flows into moulds, and in electrical steel for transformer cores. Boron in parts-per-million amounts sharply raises the hardenability of steel.

Antimony and arsenic harden lead alloys, and tellurium makes copper and steel easier to machine. These alloys make up a large share of total metalloid demand.

Flame retardants are the other big market. Antimony trioxide works with halogen-containing additives to slow combustion in plastics, cable sheaths and textiles, and boron compounds such as zinc borate are used for the same purpose, partly because they form a glassy char that shields the material underneath.

Glass, catalysts and pyrotechnics

The oxides of boron, silicon, germanium, arsenic and antimony all form glass readily. Ordinary window glass is mostly silica, borosilicate glass adds boron oxide for heat resistance, and germanium oxide glass transmits infrared light for specialist optics. Arsenic and antimony oxides have long been used to remove bubbles from molten glass.

As catalysts, boron trifluoride and related compounds speed up organic reactions in the petrochemical industry, and antimony compounds catalyse polyester production. In pyrotechnics, boron gives a green flame and is used in igniters and delay fuses, silicon serves as a fuel in delay compositions, and antimony sulfide adds brilliance to some fireworks.

Optical storage and optoelectronics

Rewritable DVDs and Blu-ray discs store data in a thin film of germanium–antimony–tellurium alloy that a laser switches between crystalline and amorphous states, each reflecting light differently. That alloy is made entirely from three of the six commonly recognised elements above, and it works because of their in-between bonding: the atoms are held loosely enough to rearrange in a few nanoseconds under a laser pulse, yet firmly enough to stay put for years.

The same phase-change behaviour is now used in some computer memory. In optoelectronics more broadly, gallium arsenide and related compounds power LEDs and laser diodes, and germanium detects infrared light in fibre networks.

Biological uses and toxicity

All of the metalloids have been used in medicine or pest control at some point. Boric acid is a mild antiseptic and insecticide, boron is an essential trace nutrient for plants, and silicon compounds appear in many biological structures, from the shells of diatoms to the stiffening in grasses. Arsenic trioxide is an approved treatment for one form of leukaemia, and antimony compounds have long been used against parasitic diseases.

Toxicity is the flip side: arsenic compounds are notorious poisons, antimony and tellurium compounds are harmful in moderate doses, and long-term exposure to fine silica dust causes the lung disease silicosis. In industry, arsenic and antimony compounds are handled under strict exposure limits.

Metalloids in manufacturing

In manufacturing, metalloids show up mainly in electronics, energy and aerospace materials. Silicon wafers are sliced, polished and patterned into chips and solar cells; boron fibres and boron carbide go into lightweight composites and armour; antimony is compounded into flame-retardant plastics before they are moulded.

Materials researchers are pushing further, with silicon anodes that let lithium batteries store more energy, silicon nanowires for sensors and new metalloid-based materials for thermoelectric cooling. Because metalloids are brittle, they are almost always machined, grown as crystals or deposited as thin films rather than forged or bent.

Abundance and extraction of metalloids

The metalloids range from the most common solid element in the earth's crust to some of the rarest elements, and that range shapes their price and how much of each the world can use.

How common each metalloid is

• Silicon: about 28% of the crust by mass, second only to oxygen, and the most common metalloid by a huge margin.
• Boron: roughly 10 parts per million, but concentrated in rich borate deposits in Turkey and the United States, which keeps it affordable.
• Arsenic and germanium: each around 1.5 to 2 parts per million.
• Antimony: about 0.2 parts per million, mined mainly from the sulfide mineral stibnite.
• Tellurium: around a thousandth of a part per million, rarer than gold or platinum, and so the rare metalloid most often cited.

Price follows abundance and purity. Ordinary metallurgical-grade silicon is cheap, but the ultra-pure grade needed for chips costs far more, and germanium and tellurium are priced as speciality materials.

Extracting and purifying boron and silicon

Most metalloids are recovered by reducing an oxide or as by-products of refining other metals. Silicon is made by heating silica with carbon in an electric arc furnace:

Impure boron can be made by reducing boron oxide with magnesium powder:

Electronic-grade silicon needs another level of purity entirely, often better than one impurity atom in a billion. The crude silicon is converted to a volatile compound such as trichlorosilane, distilled, and decomposed back to silicon.

Zone refining then finishes the job: a narrow molten zone is passed slowly along a silicon rod, impurities stay dissolved in the liquid and travel with it, and the dirty end is cut off. Germanium, arsenic, antimony and tellurium are mostly recovered from the dust and residues of zinc, copper and lead smelting, so their supply depends on how much of those metals the world produces.