Properties of metals, metalloids and nonmetals
The chemical elements can be broadly divided into metals, metalloids and nonmetals according to their shared physical and chemical properties. All metals have a shiny appearance (at least when freshly polished); are good conductors of heat and electricity; form alloys with other metals; and have at least one basic oxide. Metalloids are metallic-looking brittle solids that are either semiconductors or exist in semiconducting forms, and have amphoteric or weakly acidic oxides. Typical nometals have a dull, coloured or colourless appearance; are brittle when solid; are poor conductors of heat and electricity; and have acidic oxides. Most or some elements in each category share a range of other properties; a few elements have properties that are either anomalous given their category, or otherwise extraordinary.
Metals appear lustrous (beneath any patina); form mixtures (alloys) when combined with other metals; tend to lose or share electrons when they react with other substances; and each forms at least one predominately basic oxide.
Most metals are silvery looking, high density, relatively soft and easily deformed solids with good electrical and thermal conductivity, closely packed structures, low ionisation energies and electronegativities, and are found naturally in combined states.
Some metals appear coloured (Cu, Cs, Au), have low densities (e.g. Be, Al) or very high melting points, are liquids at or near room temperature, are brittle (e.g. Os, Bi), not easily machined (e.g. Ti, Re), or are noble (hard to oxidise) or have nonmetallic structures (Mn and Ga are structurally analogous to, respectively, white P and I).
Metals comprise the large majority of the elements, and can be subdivided into several different categories. From left to right in the periodic table, these categories include the highly reactive alkali metals; the less reactive alkaline earth metals, lanthanides and radioactive actinides; the archetypal transition metals, and the physically and chemically weak post-transition metals. Specialized subcategories such as the refractory metals and the noble metals also exist.
Metalloids are metallic looking brittle solids; tend to share electrons when they react with other substances; have weakly acidic or amphoteric oxides; and are usually found naturally in combined states.
Most are semiconductors, and moderate thermal conductors, and have structures that are more open than those of most metals.
The metalloids, as the smallest major category of elements, are not subdivided further.
Nonmetals have open structures (unless solidified from gaseous or liquid forms); tend to gain or share electrons when they react with other substances; and do not form distinctly basic oxides.
Most are gases at room temperature; have relatively low densities; are poor electrical and thermal conductors; have relatively high ionisation energies and electronegativities; form acidic oxides; and are found naturally in uncombined states in large amounts.
Some nonmetals (C, black P, S and I) are brittle solids at room temperature however these are also known in malleable, pliable or ductile forms; some other nonmetals are either highly reactive (O, F, white P, Cl) or relatively unreactive (N, black P) or noble.
From left to right in the periodic table, the nonmetals can be subdivided into the polyatomic nonmetals which, being nearest to the metalloids, show some incipient metallic character; the diatomic nonmetals, which are essentially nonmetallic; and the monatomic noble gases, which are almost completely inert.
Comparison of properties
The characteristic properties of metals and nonmetals are quite distinct, as shown in the table below. Metalloids, straddling the metal-nonmetal border, are mostly distinct from either, but in a few properties resemble one or the other, as shown in the shading of the metalloid column below and summarized in the small table at the top of this section.
Authors differ in where they divide metals from nonmetals and in whether they recognize an intermediate metalloid category. Some authors count metalloids as nonmetals with weakly nonmetallic properties.[n 1] Others count some of the metalloids as post-transition metals with weakly metallic properties.
|• nearly all are shiny and grey-white
• Cu, Cs, Au shiny and golden
|shiny and grey-white|
|intermediate to typically high||intermediate|
|solid; Rb, Cs, Fr, Ga, Hg liquid at/near stp[n 3]||solid|
|generally high, with some exceptions such as the alkali metals||lower than neighbouring metals but higher than neighbouring nonmetals|
|typically ductile and malleable||brittle|
|low to high[n 5]||low to intermediate[n 6]|
|• Most are hexagonal or cubic, or distortions thereof (Be; Zn, Cd; In; U, Np)
• Ga orthorhombic; Sn, Pa tetragonal; Sm trigonal; Hg, Bi rhombohedral; Pu monoclinic
|• B, As, Sb rhombohedral
• Si, Ge cubic
• Te hexagonal
|close-packed crystal structures; high coordination numbers||relatively open crystal structures, with medium coordination numbers|
|• intermediate to very large
• 112–298 pm (10−12 m), average 187
|• small to intermediate: B, Si, Ge, As, Sb, Te
• 87–123 pm, av. 115.5
|• around half form allotropes
• a few (e.g. grey Sn, thin-film Bi) are more metalloidal or nonmetallic than others
|• all or nearly all form allotropes
• some (e.g. red B, yellow As) are more nonmetallic than others
|medium to high||mostly intermediate; Si is high|
|nearly all positive (Pu is negative)||negative (B, Si, Ge, Te) or positive (As, Sb)|
|volume generally expands||some contract, unlike (most) metals|
|low to high||intermediate to very high|
|good to high[n 10]||intermediate to good[n 11]|
|conductivity falls gradually as temperature rises[n 13]||most behave like metals|
|s, p, d, f ||p |
|• mostly low (1–3)
• except 0(Pd); 4(Sn,Pb,Fl); 5(Bi); 6(Po)
|• nearly all have substantial band overlap
• Bi has slight band overlap (semimetal)
|• most have narrow band gap (semiconductors)
• As, Sb are semimetals
|"free" electrons (facilitating electrical and thermal conductivity)||• valence electrons less freely delocalized; considerable covalent bonding present
• have Goldhammer-Herzfeld criterion[n 14] ratios straddling unity
|about 14%, mostly Al, Na, Ng, Ca, Fe, K||about 17%, mostly Si|
|• most occur in combined states, as carbonates, silicates, phosphates, oxides, sulfides, or halides
• some (e.g. Au, Cu, Ag, Pt) occur in free or uncombined states
|all occur in combined states, as borates, silicates, sulfides, or tellurides|
|some required in large (Na, Mg, K, Ca) or trace amounts||some required in trace (B, Si, As) amounts|
|• about 1.5% Ca
• traces of most others through 92U
|trace amounts of B, Si, Ge, As, Sb, Te|
|tend to form cations||• some tendency to form anions in water;
• solution chemistry dominated by formation and reactions of oxyanions
|seldom form covalent||can form salts as well as covalent compounds|
|nearly always positive||positive or negative|
|usually low||• Pauling: close to 2
• Allen: in narrow range 1.9–2.2[n 17]
|form alloys||can form alloys|
|carbides and organometallic compounds||same as metals|
|• alkali, alkaline earth metals: form ionic, solid hydrides with high melting points;
• transition metals: metallic hydrides;
• post-transition metals: covalent hydrides
|covalent, volatile hydrides|
|• nearly all solid (Mn2O7 is a liquid)
• very few glass formers
• lower oxides: ionic and basic
• higher oxides: more covalent, acidic
• glass formers (B, Si, Ge, As, Sb, Te)
• polymeric in structure; tend to be amphoteric or weakly acidic
|do form[n 19][n 20]||most form[n 21]|
|• ionic, involatile
• mostly water soluble (not hydrolysed)
• higher halides, those of weaker metals: greater covalency and volatility, and more or less prone to hydrolysis (layer-lattice types often reversibly so) and to dissolution in organic solvents
|• covalent, volatile
• some partly reversibly hydrolysed
• usually dissolve in organic solvents
Within each category, elements can be found with one or two properties very different from the expected norm, or that are otherwise notable.
- Sodium, platinum and gold can form anions, contrary to the common ideas that "alkali metal ions (group 1A) always have a +1 charge" and that "transition elements do not form anions". The synthesis of a crystalline salt of the sodium anion Na− was reported in 1974. Since then further compounds ("alkalides") containing anions of all other alkali metals except Li and Fr, as well as that of Ba, have been prepared. In 1943, Sommer reported the preparation of the yellow transparent compound CsAu. This was subsequently shown to consist of caesium cations (Cs+) and auride anions (Au−) although it was some years before this conclusion was accepted. Several other aurides (KAu, RbAu) have since been synthesized, as well as the red transparent compound Cs2Pt which was found to contain Cs+ and Pt2− ions.
- Manganese has a complex crystal structure with a 58-atom unit cell, effectively four different atomic radii, and four different coordination numbers (10, 11, 12 and 16). It has been described as resembling "a quaternary intermetallic compound with four Mn atom types bonding as if they were different elements." The half-filled 3d shell of manganese appears to be the cause of the complexity. This confers a large magnetic moment on each atom. Below 727° C, a unit cell of 58 spatially diverse atoms represents the energetically lowest way of achieving a zero net magnetic moment. The crystal structure of manganese makes it a hard and brittle metal, with low electrical and thermal conductivity. At higher temperatures "greater lattice vibrations nullify magnetic effects" and manganese adopts less complex structures.
- Iron, cobalt, nickel, gadolinium, and dysprosium are the only elements strongly attracted to magnets: iron, cobalt, and nickel at room temperature, gadolinium just below, and dysprosium at ultra cold temperatures (below −185 °C).
- Iridium is the only element with an oxidation state of +9, in the [IrO4]+ cation. Other than this, the highest known oxidation state is +8, in Ru, Xe, Os, Ir, Pu, Cm, and Hs.
- Gold is extraordinarily malleable; a fist sized lump can be hammered and separated into one million paper back sized sheets, each 10 nm thick.
- Mercury is a liquid 13.5 times denser than water, meaning bricks and bowling balls will float on its surface. Equally, a solid mercury bowling ball would weigh around 50 pounds and, if it could be kept cold enough, would float on the surface of liquid gold. Uniquely among metals, mercury has an ionisation energy that is higher than those of the nonmetals sulfur and selenium.
- Lead is ordinarily thought of as a dense, heavy metal—being nearly as dense as mercury—giving rise to the expression, to "go down like a lead balloon." However, it is possible to construct a lead balloon, filled with a helium and air mixture, which will float and be buoyant enough to carry a small load.
- Bismuth was discovered in the Middle Ages and until recently was thought to have the highest atomic number (Z = 83) of stable elements. In 2003 it was found to be slightly radioactive: its only primordial isotope, bismuth-209, decays via alpha decay with a half life more than a billion times the estimated age of the universe. This gives lead the highest atomic number (Z = 82) of stable element.
- Plutonium increases its electrical conductivity when heated, in the temperature range of around –175 to +125 °C (metals otherwise reduce their electrical conductivity when heated).
- Boron and antimony are record holders within the field of superacid chemistry. For seven decades, fluorosulfonic acid HSO3F and trifluoromethanesulfonic acid CF3SO3H were the strongest known acids, isolable as single compounds. This record was surpassed in 2004 with the synthesis of carborane acid H(CHB11Cl11) a substance one million times more acidic than concentrated sulfuric acid. The strongest known acid, which is 10 billion times stronger than carborane acid, features another metalloid. This is fluoroantimonic acid H2F[SbF6] which is a mixture of antimony pentafluoride SbF5 and hydrofluoric acid HF.
- Silicon is the best thermal conductor among metalloids, better than most metals. Porous silicon (p-Si) is a sponge-like form of silicon, typically prepared by the electrochemical etching of silicon wafers in a hydrofluoric acid solution. Flakes of p-Si sometimes appear red; it has a band gap of 1.97–2.1 eV. The many tiny pores in porous silicon give it an enormous internal surface area, up to 1,000 m2/cm3. When exposed to an oxidant, especially a liquid oxidant, the high surface-area to volume ratio of p-Si creates a very efficient burn, accompanied by nano-explosions, and sometimes by ball-lightning-like plasmoids with, for example, a diameter of 0.1–0.8 m, a velocity of up to 0.5 m/s and a lifetime of up to 1s. The first ever spectrographic analysis of a ball lightning event (in 2012) revealed the presence of silicon, iron and calcium, these elements also being present in the soil.
- Arsenic sublimes rather than melts at standard atmospheric pressure, like the nonmetals carbon and red phosphorus.
- Antimony has a high-energy explosive form. It is prepared by the electrolysis of any of the heavier antimony trihalides (SbCl
3) in a hydrochloric acid solution at low temperature. It comprises amorphous antimony with some occluded antimony trihalide (7–20% in the case of the trichloride). When scratched, struck, powdered or heated quickly to 200° C, it "flares up, emits sparks and is converted explosively into the lower-energy, crystalline grey antimony."
- Hydrogen has two well known oxides, water (H2O) and hydrogen peroxide (H2O2). Water is a spectacular anomaly. Extrapolating from the heavier hydrogen chalcogenides, namely hydrogen sulfide H2S , hydrogen selenide H2Se, and hydrogen telluride H2Te, water should be "a foul-smelling, poisonous, inflammable gas…condensing to a nasty liquid [at] around –100° C". Instead, due to hydrogen bonding, water is "stable, potable, odorless, benign, and…indispensable to life". Less well known of the oxides of hydrogen is the trioxide, H2O3. Berthelot proposed the existence of this oxide in 1880 but his suggestion was soon forgotten as there was no way of testing it using the technology of the time. Hydrogen trioxide was prepared in 1994 by replacing the oxygen used in the industrial process for making hydrogen peroxide, with ozone. The yield is about 40 per cent, at –78° C; above around –40° C it decomposes into water and oxygen. Derivatives of hydrogen trioxide, such as F3C–O–O–O–CF3 ("bis(trifluoromethyl) trioxide") are known; these are metastable at room temperature. Mendeleev went a step further, in 1895, and proposed the existence of hydrogen tetroxide HO–O–O–OH as a transient intermediate in the decomposition of hydrogen peroxide; this was prepared and characterised in 1974, using a matrix isolation technique. Alkali metal ozonide salts of the unknown hydrogen ozonide (HO3) are also known; these have the formula MO3.
- Helium has two heavier isotopes, helium-3 and helium-4, each of which has a negative enthalpy of fusion at temperatures below 0.3 and 0.8 K, respectively. This means that, at the appropriate constant pressures, these substances freeze with the addition of heat.
- Carbon in the form of graphite is the most electrically conductive nonmetal, better than some metals. Carbon in the form of diamond is the best natural conductor of heat; it even feels cold to the touch. Its thermal conductivity (2,200 W/m•K) is five times greater than the most conductive metal (Ag at 429); 300 times higher than the least conductive metal (Pu at 6.74); and nearly 4,000 times that of water (0.58) and 100,000 times that of air (0.0224). This high thermal conductivity is used by jewelers and gemologists to separate diamonds from imitations. Graphene aerogel, produced in 2012 by freeze-drying a solution of carbon nanotubes and graphite oxide sheets, and chemically removing oxygen, is seven times lighter than air, and ten per cent lighter than helium. It is the lightest solid known (0.16 mg/cm3), conductive and elastic.
- Phosphorus in its white form is the least stable and most reactive form of the element. It is a hazardous, highly flammable and toxic substance, spontaneously igniting in air and producing phosphoric acid residue. It is therefore normally stored under water. White phosphorus is also the most common, industrially important, and easily reproducible allotrope, and for these reasons is regarded as the standard state of phosphorus. The most stable form is the black allotrope, which is a metallic looking, brittle and relatively non-reactive semiconductor (unlike the white allotrope, which has a white or yellowish appearance, is pliable, highly reactive and a semiconductor). When assessing periodicity in the physical properties of the elements it needs to be borne in mind that the quoted properties of phosphorus tend to be those of its least stable form rather than, as is the case with all other elements, the most stable form.
- Iodine is the mildest halogen and is used as a disinfectant in the form of tincture of iodine, which can be found in household medicine cabinets or emergency survival kits and will rapidly dissolve gold. Such a task ordinarily necessitates the use of aqua regia, a highly corrosive mixture of nitric and hydrochloric acids.
- For example:
- Brinkley writes that boron has weakly nonmetallic properties.
- Glinka describes silicon as a weak nonmetal.
- Eby et al. discuss the weak chemical behaviour of the elements close to the metal-nonmetal borderline.
- Booth and Bloom say "A period represents a stepwise change from elements strongly metallic to weakly metallic to weakly nonmetallic to strongly nonmetallic, and then, at the end, to an abrupt cessation of almost all chemical properties ...".
- Cox notes "nonmetallic elements close to the metallic borderline (Si, Ge, As, Sb, Se, Te) show less tendency to anionic behaviour and are sometimes called metalloids."
- At standard pressure and temperature, unless otherwise noted
- Copernicium is reported to be the only metal known to be a gas at room temperature.
- For polycrystalline forms of the elements unless otherwise noted. Determining Poisson's ratio accurately is a difficult proposition and there could be considerable uncertainty in some reported values.
- Beryllium has the lowest known value (0.0476) amongst elemental metals; indium and thallium each have the highest known value (0.46). Around one third show a value ≥ 0.33.
- Boron 0.13; silicon 0.22; germanium 0.278; amorphous arsenic 0.27; antimony 0.25; tellurium ~0.2.
- Graphitic carbon 0.25; [diamond 0.0718]; black phosphorus 0.30; sulfur 0.287; amorphous selenium 0.32; amorphous iodine ~0.
- At atmospheric pressure, for elements with known structures
- At or near room temperature
- Metals have electrical conductivity values of from 6.9 × 103 S•cm−1 for manganese to 6.3 × 105 for silver.
- Metalloids have electrical conductivity values of from 1.5 × 10−6 S•cm−1 for boron to 3.9 × 104 for arsenic. If selenium is included as a metalloid the applicable conductivity range would start from ~10−9 to 10−12 S•cm−1.
- Nonmetals have electrical conductivity values of from ~10−18 S•cm−1 for the elemental gases to 3 × 104 in graphite.
- Mott and Davis note however that 'liquid europium has a negative temperature coefficient of resistance' i.e. that conductivity increases with rising temperature
- The Goldhammer-Herzfeld criterion is a ratio that compares the force holding an individual atom's valence electrons in place with the forces, acting on the same electrons, arising from interactions between the atoms in the solid or liquid element. When the interatomic forces are greater than or equal to the atomic force, valence electron itinerancy is indicated. Metallic behaviour is then predicted. Otherwise nonmetallic behaviour is anticipated. The Goldhammer-Herzfeld criterion is based on classical arguments. It nevertheless offers a relatively simple first order rationalization for the occurrence of metallic character amongst the elements.
- Hydrogen gas is produced by some bacteria and algae and is a natural component of flatus. It can be found in the Earth's atmosphere at a concentration of 1 part per million by volume.
- Fluorine can be found in its elemental form, as an occlusion in the mineral antozonite
- Chedd defines metalloids as having electronegativity values of 1.8 to 2.2 (Allred-Rochow scale). He included boron, silicon, germanium, arsenic, antimony, tellurium, polonium and astatine in this category. In reviewing Chedd's work, Adler described this choice as arbitrary, given other elements have electronegativities in this range, including copper, silver, phosphorus, mercury and bismuth. He went on to suggest defining a metalloid simply as, 'a semiconductor or semimetal' and 'to have included the interesting materials bismuth and selenium in the book'.
- Phosphorus is known to form a carbide in thin films.
- See, for example, the sulfates of the transition metals, the lanthanides and the actinides.
- Sulfates of osmium have not been characterized with any great degree of certainty.
- Common metalloids: Boron is reported to be capable of forming an oxysulfate (BO)2SO4, a bisulfate B(HSO4)3 and a sulfate B2(SO4)3. The existence of a sulfate has been disputed. In light of the existence of silicon phosphate, a silicon sulfate might also exist. Germanium forms an unstable sulfate Ge(SO4)2 (d 200 °C). Arsenic forms oxide sulfates As2O(SO4)2 (= As2O3.2SO3) and As2(SO4)3 (= As2O3.3SO3). Antimony forms a sulfate Sb2(SO4)3 and an oxysulfate (SbO)2SO4. Tellurium forms an oxide sulfate Te2O3(SO)4. Less common: Polonium forms a sulfate Po(SO4)2. It has been suggested that the astatine cation forms a weak complex with sulfate ions in acidic solutions.
- Hydrogen forms hydrogen sulfate H2SO4. Carbon forms (a blue) graphite hydrogen sulfate C+
4 • 2.4H2SO4. Nitrogen forms nitrosyl hydrogen sulfate (NO)HSO4 and nitronium (or nitryl) hydrogen sulfate (NO2)HSO4. There are indications of a basic sulfate of selenium SeO2.SO3 or SeO(SO4). Iodine forms a polymeric yellow sulfate (IO)2SO4.
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|Periodic table (Large cells)|