# 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 elemental metals have a shiny appearance (at least when freshly polished); are good conductors of heat and electricity; form alloys with other metallic elements; and have at least one basic oxide. Metalloids are metallic-looking, often brittle solids that are either semiconductors or exist in semiconducting forms, and have amphoteric or weakly acidic oxides. Typical elemental nonmetals have a dull, coloured or colourless appearance; are often 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.

## Properties
### Metals

Elemental metals appear lustrous (beneath any patina); form compounds (alloys) when combined with other elements; tend to lose or share electrons when they react with other substances; and each forms at least one predominantly 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 (e.g. W, Nb), are liquids at or near room temperature (e.g. Hg, Ga), are brittle (e.g. Os, Bi), not easily machined (e.g. Ti, Re), or are noble (hard to oxidise, e.g. Au, Pt), 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

Metalloids are metallic-looking often 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.
Some metalloids (As, Sb) conduct electricity like metals.
The metalloids, as the smallest major category of elements, are not subdivided further.

### Nonmetals

Nonmetallic elements often have open structures; 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 (black P, S, and Se) are brittle solids at room temperature (although each of these also have malleable, pliable or ductile allotropes).
From left to right in the periodic table, the nonmetals can be divided into the reactive nonmetals and the noble gases. The reactive nonmetals near the metalloids show some incipient metallic character, such as the metallic appearance of graphite, black phosphorus, selenium and iodine. The noble gases are almost completely inert.

## Comparison of properties
### Overview

The characteristic properties of elemental 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. Others count some of the metalloids as post-transition metals.

### Details

Physical and chemical propertiesvte :
| | Metals | Metalloids | Nonmetals |
| Form and structure | Form and structure | Form and structure | |
| Colour | nearly all are shiny and grey-whiteCu, Cs, Au: shiny and golden | shiny and grey-white | most are colourless or dull red, yellow, green, or intermediate shadesC, P, Se, I: shiny and grey-white |
| Reflectivity | intermediate to typically high | intermediate | zero or low (mostly) to intermediate |
| Form | almost all solidRb, Cs, Fr, Ga, Hg: liquid at/near stp | all solid | most are gasesC, P, S, Se, I: solid; Br: liquid |
| Density | generally high, with some exceptions such as the alkali metals | lower than nearby metals but higher than nearby nonmetals | often low |
| Deformability (as a solid) | most are ductile and malleablesome are brittle (Cr, Mn, Ga, Ru, W, Os, Bi) | often brittle | often brittlesome (C, P, S, Se) have non-brittle forms |
| Poisson's ratio | low to high | low to intermediate | low to intermediate |
| Crystalline structure at freezing point | most are hexagonal or cubicGa, U, Np: orthorhombic; In, Sn, Pa: tetragonal; Sm, Hg, Bi: rhombohedral; Pu: monoclinic | B, As, Sb: rhombohedralSi, Ge: cubicTe: hexagonal | H, He, C, N, Se: hexagonalO, F, Ne, P, Ar, Kr, Xe, Rn: cubicS, Cl, Br, I: orthorhombic |
| Packing & coordination number | close-packed crystal structureshigh coordination numbers | relatively open crystal structuresmedium coordination numbers | open structureslow coordination numbers |
| Atomic radius (calculated) | intermediate to very large112–298 pm, average 187 | small to intermediate: B, Si, Ge, As, Sb, Te87–123 pm, average 115.5 pm | very small to intermediate31–120 pm, average 76.4 pm |
| Allotropes | around half form allotropesone (Sn) has a metalloid-like allotrope (grey Sn, which forms below 13.2 °C) | all or nearly all form allotropessome (e.g. red B, yellow As) are more nonmetallic in nature | some form allotropessome (e.g. graphitic C, black P, grey Se) are more metalloidal or metallic in nature |
| Electron-related ||||
| Periodic table block | s, p, d, f | p | s, p |
| Outer s and p electrons | few in number (1–3)except 0 (Pd); 4 (Sn, Pb, Fl); 5 (Bi); 6 (Po) | medium number (3–7) | high number (4–8)except 1 (H); 2 (He) |
| Electron bands: (valence, conduction) | nearly all have substantial band overlapBi: has slight band overlap (semimetal) | most have narrow band gap (semiconductors)As, Sb are semimetals | most have wide band gap (insulators)C (graphite): a semimetalP (black), Se, I: semiconductors |
| Electron behaviour | "free" electrons (facilitating electrical and thermal conductivity) | valence electrons less freely delocalized; considerable covalent bonding presenthave Goldhammer-Herzfeld criterion ratios straddling unity | no, few, or directionally confined "free" electrons (generally hampering electrical and thermal conductivity) |
| Electrical conductivity | good to high | intermediate to good | poor to good |
| ... as a liquid | falls gradually as temperature rises | most behave like metals | increases as temperature rises |
| Thermodynamics ||||
| Thermal conductivity | medium to high | mostly intermediate; Si is high | almost negligible to very high |
| Temperature coefficient of resistance | nearly all positive (Pu is negative) | negative (B, Si, Ge, Te) or positive (As, Sb) | nearly all negative (C, as graphite, is positive in the direction of its planes) |
| Melting point | mostly high | mostly high | mostly low |
| Melting behaviour | volume generally expands | some contract, unlike (most) metals | volume generally expands |
| Enthalpy of fusion | low to high | intermediate to very high | very low to low (except C: very high) |
| Elemental chemistry ||||
| Overall behaviour | metallic | nonmetallic | nonmetallic |
| Ion formation | tend to form cations | some tendency to form anions in watersolution chemistry dominated by formation and reactions of oxyanions | tend to form anions |
| Bonds | seldom form covalent compounds | form salts as well as covalent compounds | form many covalent compounds |
| Oxidation number | nearly always positive | positive or negative | positive or negative |
| Ionization energy | relatively low | intermediate | high |
| Electronegativity | usually low | close to 2, i.e., 1.9–2.2 | high |
| Combined form chemistry ||||
| With metals | form alloys | can form alloys | form ionic or interstitial compounds |
| With carbon | carbides and organometallic compounds | same as metals | carbon-nonmetal (e.g. CO$_2$, CS$_2$) or organic (e.g. CH$_4$, C$_6$H$_{12}$O$_6$) compounds |
| With hydrogen (hydrides) | ionic, with alkali metals, alkaline earth metalsmetallic, with transition metalscovalent, with post-transition metals | covalent, volatile hydrides | covalent, gaseous or liquid hydrides |
| With oxygen (oxides) | nearly all solid (Mn$_2$O$_7$ is a liquid)very few glass formerslower oxides: ionic and basichigher oxides: more covalent, acidic | solidglass formers (B, Si, Ge, As, Sb, Te)polymeric in structure; tend to be amphoteric or weakly acidic | solid, liquid or gaseousfew glass formers (P, S, Se)covalent, acidic |
| With sulfur (sulfates) | do form | most form | some form |
| With halogens (halides, esp. chlorides) (see also) | typically ionic, involatilegenerally insoluble in organic solventsmostly water-soluble (not hydrolysed)more covalent, volatile, and susceptible to hydrolysis and organic solvents with higher halogens and weaker metals | covalent, volatileusually dissolve in organic solventspartly or completely hydrolysedsome reversibly hydrolysed | covalent, volatileusually dissolve in organic solventsgenerally completely or extensively hydrolyzednot always susceptible to hydrolysis if parent nonmetal at maximum covalency for period e.g. CF$_4$, SF$_6$ (then nil reaction) |
| Environmental chemistry ||||
| Molar composition of Earth's ecosphere | about 14%, mostly Al, Na, Mg, Ca, Fe, K | about 17%, mostly Si | about 69%, mostly O, H |
| Primary form on Earth | most occur in combined states, as carbonates, silicates, phosphates, oxides, sulfides, or halidessome (e.g. Au, Cu, Ag, Pt) occur in free or uncombined states | all occur in combined states, as borates, silicates, sulfides, or tellurides | elemental C, N, O, S, noble gases are plentifulH, F, Se occur primarily in compoundsP, Cl, Br, I occur only in compounds, as phosphates, oxides, selenides or halides |
| Required by mammals | large amounts needed: Na, Mg, K, Catrace amounts needed of some others | trace amounts needed: B, Si, As | large amounts needed: H, C, N, O, P, S, Cltrace amounts needed: Se, Br, I, possibly Fonly noble gases not needed |
| Composition of the human body, by weight | about 1.5% Catraces of most others through $_{92}$U | trace amounts of B, Si, Ge, As, Sb, Te | about 97% O, C, H, N, Pothers detectable except noble gases |

## Anomalous properties

   There were exceptions... in the periodic table, anomalies too—some of them profound. Why, for example, was manganese such a bad conductor of electricity, when the elements on either side of it were reasonably good conductors? Why was strong magnetism confined to the iron metals? And yet these exceptions, I was somehow convinced, reflected special additional mechanisms at work...
Within each category, elements can be found with one or two properties very different from the expected norm, or that are otherwise notable.

### Metals

Sodium, potassium, rubidium, caesium, barium, platinum, gold
   * The common notions that "alkali metal ions (group 1A) always have a +1 charge" and that "transition elements do not form anions" are textbook errors. 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 Cs$_2$Pt which was found to contain Cs$^+$ and Pt$^{2−}$ ions.
Manganese
   * Well-behaved metals have crystal structures featuring unit cells with up to four atoms. 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, terbium, dysprosium, holmium, erbium, thulium
   * The only elements strongly attracted to magnets are iron, cobalt, and nickel at room temperature, gadolinium just below, and terbium, dysprosium, holmium, erbium, and thulium at ultra-cold temperatures (below −54 °C, −185 °C, −254 °C, −254 °C, and −241 °C respectively).
Iridium
   * The only element encountered with an oxidation state of +9 is iridium, in the [IrO$_4$]$^+$ cation. Other than this, the highest known oxidation state is +8, in Ru, Xe, Os, Ir, and Hs.
Gold
   * The malleability of gold is extraordinary: a fist-sized lump can be hammered and separated into one million paperback-sized sheets, each 10 nm thick, 1600 times thinner than regular kitchen aluminium foil (0.016 mm thick).
Mercury
* Bricks and bowling balls will float on the surface of mercury thanks to it having a density 13.5 times that of water. 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.
* The only metal having an ionisation energy higher than some nonmetals (sulfur and selenium) is mercury.
* Mercury and its compounds have a reputation for toxicity but on a scale of 1 to 10, dimethylmercury ((CH$_3$)$_2$Hg) (abbr. DMM), a volatile colourless liquid, has been described as a 15. It is so dangerous that scientists have been encouraged to use less-toxic mercury compounds wherever possible. In 1997, Karen Wetterhahn, a professor of chemistry specialising in toxic metal exposure, died of mercury poisoning ten months after a few drops of DMM landed on her "protective" latex gloves. Although Wetterhahn had been following the then-published procedures for handling this compound, it passed through her gloves and skin within seconds. It is now known that DMM is exceptionally permeable to (ordinary) gloves, skin, and tissues. And its toxicity is such that less than one-tenth of a ml applied to the skin will be seriously toxic.
Lead
   * The expression, to "go down like a lead balloon" is anchored in the common view of lead as a dense, heavy metal—being nearly as dense as mercury. However, it is possible to construct a balloon made of lead foil, filled with a helium and air mixture, which will float and be buoyant enough to carry a small load.
Bismuth
   * Bismuth has the longest half-life of any naturally occurring element; its only primordial isotope, bismuth-209, was found in 2003 to be slightly radioactive, decaying via alpha decay with a half-life more than a billion times the estimated age of the universe. Prior to this discovery, bismuth-209 was thought to be the heaviest naturally occurring stable isotope; this distinction now belongs to lead-208.
Uranium
   * The only element with a naturally occurring isotope capable of undergoing nuclear fission is uranium. The capacity of uranium-235 to undergo fission was first suggested (and ignored) in 1934, and subsequently discovered in 1938.
Plutonium
   * It is normally true that metals reduce their electrical conductivity when heated. Plutonium increases its electrical conductivity when heated in the temperature range of around –175 to +125 °C. There is evidence that this behavior, and similar with some of the other transuranic elements is due to more complex relativistic and spin interactions that are not captured by simple model of electrical conductivity.

### Metalloids

Boron
   * Boron is the only element with a partially disordered structure in its most thermodynamically stable crystalline form.
Boron, antimony
   * These elements are record holders within the field of superacid chemistry. For seven decades, fluorosulfonic acid HSO$_3$F and trifluoromethanesulfonic acid CF$_3$SO$_3$H were the strongest known acids that could be isolated as single compounds. Both are about a thousand times more acidic than pure sulfuric acid. In 2004, a boron compound broke this record by a thousand fold with the synthesis of carborane acid H(CHB$_{11}$Cl$_{11}$). Another metalloid, antimony, features in the strongest known acid, a mixture 10 billion times stronger than carborane acid. This is fluoroantimonic acid H$_2$F[SbF$_6$], a mixture of antimony pentafluoride SbF$_5$ and hydrofluoric acid HF.
Silicon
* The thermal conductivity of silicon is better than that of most metals.
* A sponge-like porous form of silicon (p-Si) is 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 m$^2$/cm$^3$. 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
   * Metals are said to be fusible, resulting in some confusion in old chemistry as to whether arsenic was a true metal, or a nonmetal, or something in-between. It sublimes rather than melts at standard atmospheric pressure, like the nonmetals carbon and red phosphorus.
Antimony
   * A high-energy explosive form of antimony was first produced in 1858. It is prepared by the electrolysis of any of the heavier antimony trihalides (SbCl$_3$, SbBr$_3$, SbI$_3$) 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".

### Nonmetals

Hydrogen
* Water (H$_2$O), a well-known oxide of hydrogen, is a spectacular anomaly. Extrapolating from the heavier hydrogen chalcogenides, namely hydrogen sulfide H$_2$S, hydrogen selenide H$_2$Se, and hydrogen telluride H$_2$Te, 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, H$_2$O$_3$. 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 F$_3$C–O–O–O–CF$_3$ ("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 (HO$_3$) are also known; these have the formula MO$_3$.
Helium
* At temperatures below 0.3 and 0.8 K respectively, helium-3 and helium-4 each have a negative enthalpy of fusion. This means that, at the appropriate constant pressures, these substances freeze with the addition of heat.
* Until 1999 helium was thought to be too small to form a cage clathrate—a compound in which a guest atom or molecule is encapsulated in a cage formed by a host molecule—at atmospheric pressure. In that year the synthesis of microgram quantities of He@C$_{20}$H$_{20}$ represented the first such helium clathrate and (what was described as) the world's smallest helium balloon.
Carbon
* Graphite is the most electrically conductive nonmetal, better than some metals.
* 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/cm$^3$), conductive and elastic.
Phosphorus
   * The least stable and most reactive form of phosphorus is the white allotrope. 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
   * The mildest of the halogens, iodine is the active ingredient in tincture of iodine, a disinfectant. This can be found in household medicine cabinets or emergency survival kits. Tincture of iodine will rapidly dissolve gold, a task ordinarily requiring the use of aqua regia (a highly corrosive mixture of nitric and hydrochloric acids).