Properties of metals, metalloids and nonmetals

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The

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

Pure (99.97%+) iron chips, electrolytically refined, accompanied by a high-purity (99.9999% = 6N) 1 cm3 cube
Main article: Metal

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 predominantly basic oxide.

Most metals are silvery looking, high density, relatively soft and easily deformed solids with good

ionisation energies and electronegativities
, and are found naturally in combined states.

Some metals appear coloured (

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

refractory metals and the noble metals
also exist.

Metalloids

A shiny silver-white medallion with a striated surface, irregular around the outside, with a square spiral-like pattern in the middle
Tellurium, described by Dmitri Mendeleev as forming a transition between metals and nonmetals[1]
Main article: Metalloid

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.

Some metalloids (As, Sb) conduct electricity like metals.

The metalloids, as the smallest major category of elements, are not subdivided further.

Nonmetals

25 ml of bromine, a dark red-brown liquid at room temperature
Main article:
Nonmetal (chemistry)

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 (

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

Number of metalloid properties that resemble metals or nonmetals
(or that are relatively distinct)
     Resemble metals        Relatively distinct     Resemble nonmetals  
Properties compared: (37)   7 (19%) 25  (68%) 5 (13%) 
Physical properties (21)   5 (24%) 14  (67%) 2 (10%) 
 • Form & structure (10)   2 6  2 (20%) 
 • Electron-related (6)   1 5 
 • Thermodynamics (5)   2 3 
Chemical properties (16)   2 (13%) 11  (69%) 3 (19%) 
 • Elemental chemistry (6)   3  3 (50%) 
 • Combined form chemistry (6)   2 4 
 • Environmental chemistry (4)  4 
                                                                                                                                                                                                       

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.[n 2]

Details

Physical and chemical properties
t
  • e
    • Metals[8] Metalloids Nonmetals[8]
    Form and structure
    Colour
    • nearly all are shiny and grey-white
    • Cu, Cs, Au: shiny and golden[9]
    • shiny and grey-white[10]
    • most are colourless or dull red, yellow, green, or intermediate shades[11]
    • C, P, Se, I: shiny and grey-white
    Reflectivity
    • zero or low (mostly)[16] to intermediate[17]
    Form
    Density
    • lower than nearby
      nonmetals[23]
    • often low
    Deformability (as a solid)
    • brittle, when solid
    • some (C, P, S, Se) have non-brittle forms[n 6]
    Poisson's ratio[n 7]
    Crystalline structure at freezing point[47]
    : monoclinic
    Packing & coordination number
    • close-packed crystal structures[48]
    • high coordination numbers
    • relatively open crystal structures[49]
    • medium coordination numbers[50]
    • open structures[51]
    • low coordination numbers
    Atomic radius
    (calculated)[52]
    • intermediate to very large
    • 112–298 pm, average 187
    • small to intermediate: B, Si, Ge, As, Sb, Te
    • 87–123 pm, average 115.5 pm
    • very small to intermediate
    • 31–120 pm, average 76.4 pm
    Allotropes[53][n 11]
    • around half form allotropes
    • one (Sn) has a metalloid-like allotrope (grey Sn, which forms below 13.2 °C[54])
    • all or nearly all form allotropes
    • some (e.g. red B, yellow As) are more nonmetallic in nature
    Electron-related
    Periodic table block
    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)
    conduction
    )
    • nearly all have substantial band overlap
    • Bi: has slight band overlap (semimetal)
    Electron behaviour
    • "free" electrons (facilitating electrical and thermal conductivity)
    • valence electrons less freely delocalized; considerable covalent bonding present[57]
    • have Goldhammer-Herzfeld criterion[n 12] ratios straddling unity[61][62]
    • no, few, or directionally confined "free" electrons (generally hampering electrical and thermal conductivity)
    Electrical conductivity
    ... as a liquid[70]
    • falls gradually as temperature rises[n 16]
    • increases as temperature rises
    Thermodynamics
    Thermal conductivity
    Temperature coefficient of resistance[n 17]
    • nearly all positive (Pu is negative)[77]
    • nearly all negative (C, as graphite, is positive in the direction of its planes)[80][81]
    Melting point
    • mostly high
    • mostly high
    • mostly low
    Melting behaviour
    • volume generally expands[82]
    • some contract, unlike (most)[83] metals[84]
    • volume generally expands[82]
    Enthalpy of fusion
    • low to high
    • intermediate to very high
    • very low to low (except C: very high)
    Elemental chemistry
    Overall behaviour
    • metallic
    • nonmetallic
    Ion formation
    • tend to form
      cations
    • some tendency to form
      anions in water[6]
    • solution chemistry dominated by formation and reactions of oxyanions[86][87]
    • tend to form anions
    Bonds
    • seldom form covalent compounds
    • form many covalent compounds
    Oxidation number
    • nearly always positive
    • positive or negative[89]
    • positive or negative
    Ionization energy
    • relatively low
    • high
    Electronegativity
    • usually low
    • high
    Combined form chemistry
    With metals
    • form
      interstitial
      compounds
    With carbon
    • organometallic compounds
    • same as metals
    With hydrogen (hydrides)
    • covalent, volatile hydrides[98]
    • covalent, gaseous or liquid hydrides
    With oxygen (oxides)
    • solid
    • glass formers (
      amphoteric or weakly acidic[10][102]
    • solid, liquid or gaseous
    • few glass formers (P, S, Se)[103]
    • covalent, acidic
    With sulfur (sulfates)
    With halogens (halides, esp. chlorides) (see also[124])
    • typically ionic, involatile
    • generally insoluble in organic solvents
    • mostly water-soluble (not hydrolysed)
    • more covalent, volatile, and susceptible to hydrolysis[n 24] and organic solvents with higher halogens and weaker metals[125][126]
    • covalent, volatile[127]
    • usually dissolve in organic solvents[128]
    • partly or completely hydrolysed[129]
    • some reversibly hydrolysed[129]
    • covalent, volatile
    • usually dissolve in organic solvents
    • generally completely or extensively hydrolyzed
    • not always susceptible to hydrolysis if parent nonmetal at maximum covalency for period e.g. CF4, SF6 (then nil reaction)[130]
    Environmental chemistry
    Molar composition of Earth's ecosphere[n 25]
    • about 14%, mostly Al, Na, Mg, Ca, Fe, K
    • about 17%, mostly Si
    • about 69%, mostly O, H
    Primary form on Earth
    Required by mammals
    • large amounts needed: Na, Mg, K, Ca
    • trace amounts needed of some others
    • trace amounts needed: B, Si, As
    • large amounts needed: H, C, N, O, P, S, Cl
    • trace amounts needed: Se, Br, I, possibly F
    • only noble gases not needed
    Composition of the human body, by weight
    • about 1.5% Ca
    • traces of most others through 92U
    • about 97% O, C, H, N, P
    • others 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...

    Oliver Sacks
    Uncle Tungsten (2001, p. 204)

    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"[136] and that "transition elements do not form anions"[137] 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 Cs2Pt which was found to contain Cs+ and Pt2− ions.[138]

    Manganese

    • Well-behaved metals have crystal structures featuring
      intermetallic compound with four Mn atom types bonding as if they were different elements."[139] 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.[140] 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"[139] and manganese adopts less-complex structures.[141]

    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).[142]

    Iridium

    • The only element encountered with an oxidation state of +9 is iridium, in the [IrO4]+ cation. Other than this, the highest known oxidation state is +8, in Ru, Xe, Os, Ir, and Hs.[143]

    Gold

    Mercury

    1. 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.[citation needed]
    2. The only metal having an ionisation energy higher than some nonmetals (sulfur and selenium) is mercury.[citation needed]
    3. Mercury and its compounds have a reputation for toxicity but on a scale of 1 to 10, dimethylmercury ((CH3)2Hg) (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.[144]

    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.[citation needed]

    Bismuth

    Uranium

    • The only element with a naturally occurring isotope capable of undergoing nuclear fission is uranium.[146] The capacity of uranium-235 to undergo fission was first suggested (and ignored) in 1934, and subsequently discovered in 1938.[n 28]

    Plutonium

    • It is a commonly held belief 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.[citation needed]

    Metalloids

    Boron

    • Boron is the only element with a partially disordered structure in its most thermodynamically stable crystalline form.[149]

    Boron, antimony

    • These elements are record holders within the field of
      trifluoromethanesulfonic acid CF3SO3H 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(CHB11Cl11). Another metalloid, antimony, features in the strongest known acid, a mixture 10 billion times stronger than carborane acid. This is fluoroantimonic acid H2F[SbF6], a mixture of antimony pentafluoride SbF5 and hydrofluoric acid HF.[citation needed
      ]

    Silicon

    1. The thermal conductivity of silicon is better than that of most metals.[citation needed]
    2. A sponge-like
      oxidant,[154] especially a liquid oxidant,[153] the high surface-area to volume ratio of p-Si creates a very efficient burn, accompanied by nano-explosions,[150] 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.[155] 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.[156]

    Arsenic

    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 (SbCl3, SbBr3, SbI3) 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".[157]

    Nonmetals

    Hydrogen

    1. hydrogen bonding, water is "stable, potable, odorless, benign, and... indispensable to life".[159]
    2. Less well-known of the oxides of hydrogen is the
      hydrogen tetroxide HO–O–O–OH as a transient intermediate in the decomposition of hydrogen peroxide;[160] this was prepared and characterised in 1974, using a matrix isolation technique.[citation needed] Alkali metal ozonide salts of the unknown hydrogen ozonide (HO3) are also known; these have the formula MO3.[162]

    Helium

    1. 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.[citation needed]
    2. 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@C20H20 represented the first such helium clathrate and (what was described as) the world's smallest helium balloon.[163]

    Carbon

    1. Graphite is the most electrically conductive nonmetal, better than some metals.[citation needed]
    2. 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.[citation needed]
    3. 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.[164]

    Phosphorus

    • The least stable and most reactive form of phosphorus is the
      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.[citation needed
      ]

    Iodine

    Notes

    1. ^ For example:
      • Brinkley[2] writes that boron has weakly nonmetallic properties.
      • Glinka[3] describes silicon as a weak nonmetal.
      • Eby et al.[4] discuss the weak chemical behaviour of the elements close to the metal-nonmetal borderline.
      • Booth and Bloom[5] 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[6] 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."
    2. ^ See, for example, Huheey, Keiter & Keiter[7] who classify Ge and Sb as post-transition metals.
    3. ^ At standard pressure and temperature, for the elements in their most thermodynamically stable forms, unless otherwise noted
    4. ^ Copernicium is reported to be the only metal known to be a gas at room temperature.[20]
    5. ^ Whether polonium is ductile or brittle is unclear. It is predicted to be ductile based on its calculated elastic constants.[25] It has a simple cubic crystalline structure. Such a structure has few slip systems and "leads to very low ductility and hence low fracture resistance".[26]
    6. ^ Carbon as exfoliated (expanded) graphite,[28] and as metre-long carbon nanotube wire;[29] phosphorus as white phosphorus (soft as wax, pliable and can be cut with a knife, at room temperature);[30] sulfur as plastic sulfur;[31] and selenium as selenium wires.[32]
    7. ^ 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.[33]
    8. ^ Beryllium has the lowest known value (0.0476) among elemental metals; indium and thallium each have the highest known value (0.46). Around one third show a value ≥ 0.33.[34]
    9. ^ Boron 0.13;[35] silicon 0.22;[36] germanium 0.278;[37] amorphous arsenic 0.27;[38] antimony 0.25;[39] tellurium ~0.2.[40]
    10. ^ Graphitic carbon 0.25;[41] [diamond 0.0718];[42] black phosphorus 0.30;[43] sulfur 0.287;[44] amorphous selenium 0.32;[45] amorphous iodine ~0.[46]
    11. ^ At atmospheric pressure, for elements with known structures
    12. ^ 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.[58] Otherwise nonmetallic behaviour is anticipated. The Goldhammer-Herzfeld criterion is based on classical arguments.[59] It nevertheless offers a relatively simple first order rationalization for the occurrence of metallic character among the elements.[60]
    13. ^ Metals have electrical conductivity values of from 6.9 × 103 S•cm−1 for manganese to 6.3 × 105 for silver.[63]
    14. ^ Metalloids have electrical conductivity values of from 1.5 × 10−6 S•cm−1 for boron to 3.9 × 104 for arsenic.[65] If selenium is included as a metalloid the applicable conductivity range would start from ~10−9 to 10−12 S•cm−1.[66][67][68]
    15. ^ Nonmetals have electrical conductivity values of from ~10−18 S•cm−1 for the elemental gases to 3 × 104 in graphite.[69]
    16. ^ Mott and Davis[71] note however that 'liquid europium has a negative temperature coefficient of resistance' i.e. that conductivity increases with rising temperature
    17. ^ At or near room temperature
    18. Allred-Rochow scale). He included boron, silicon, germanium, arsenic, antimony, tellurium, polonium and astatine in this category. In reviewing Chedd's work, Adler[95] 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'.
    19. ^ Phosphorus is known to form a carbide in thin films.
    20. ^ See, for example, the sulfates of the transition metals,[104] the lanthanides[105] and the actinides.[106]
    21. ^ Sulfates of osmium have not been characterized with any great degree of certainty.[107]
    22. ^ Common metalloids: Boron is reported to be capable of forming an oxysulfate (BO)2SO4,[108] a bisulfate B(HSO4)3[109] and a sulfate B2(SO4)3.[110] The existence of a sulfate has been disputed.[111] In light of the existence of silicon phosphate, a silicon sulfate might also exist.[112] Germanium forms an unstable sulfate Ge(SO4)2 (d 200 °C).[113] Arsenic forms oxide sulfates As2O(SO4)2 (= As2O3.2SO3)[114] and As2(SO4)3 (= As2O3.3SO3).[115] Antimony forms a sulfate Sb2(SO4)3 and an oxysulfate (SbO)2SO4.[116] Tellurium forms an oxide sulfate Te2O3(SO)4.[117] Less common: Polonium forms a sulfate Po(SO4)2.[118] It has been suggested that the astatine cation forms a weak complex with sulfate ions in acidic solutions.[119]
    23. hydrogen sulfate H2SO4. Carbon forms (a blue) graphite hydrogen sulfate C+
      24      HSO–
      4       • 2.4H2SO4.[120]       Nitrogen forms nitrosyl hydrogen sulfate (NO)HSO4 and nitronium (or nitryl) hydrogen sulfate (NO2)HSO4.[121] There are indications of a basic sulfate of selenium SeO2.SO3 or SeO(SO4).[122] Iodine forms a polymeric yellow sulfate (IO)2SO4.[123]
    24. ^ layer-lattice types often reversibly so
    25. ^ Based on a table of the elemental composition of the biosphere, and lithosphere (crust, atmosphere, and seawater) in Georgievskii,[131] and the masses of the crust and hydrosphere give in Lide and Frederikse.[132] The mass of the biosphere is negligible, having a mass of about one billionth that of the lithosphere.[citation needed] "The oceans constitute about 98 percent of the hydrosphere, and thus the average composition of the hydrosphere is, for all practical purposes, that of seawater."[133]
    26. flatus
      . It can be found in the Earth's atmosphere at a concentration of 1 part per million by volume.
    27. ^ Fluorine can be found in its elemental form, as an occlusion in the mineral antozonite[135]
    28. atomic bomb and, 'the history of the world would have been [very] different.'[148]

    Citations

    1. ^ Mendeléeff 1897, p. 274
    2. ^ Brinkley 1945, p. 378
    3. ^ Glinka 1965, p. 88
    4. ^ Eby et al. 1943, p. 404
    5. ^ Booth & Bloom 1972, p. 426
    6. ^ a b Cox 2004, p. 27
    7. ^ Huheey, Keiter & Keiter 1993, p. 28
    8. ^ a b Kneen, Rogers & Simpson, 1972, p. 263. Columns 2 (metals) and 4 (nonmetals) are sourced from this reference unless otherwise indicated.
    9. ^ Russell & Lee 2005, p. 147
    10. ^ a b c Rochow 1966, p. 4
    11. ^ Pottenger & Bowes 1976, p. 138
    12. ^ Askeland, Fulay & Wright 2011, p. 806
    13. ^ Born & Wolf 1999, p. 746
    14. ^ Lagrenaudie 1953
    15. ^ Rochow 1966, pp. 23, 25
    16. ^ Burakowski & Wierzchoń 1999, p. 336
    17. ^ Olechna & Knox 1965, pp. A991‒92
    18. ^ Stoker 2010, p. 62
    19. ^ Chang 2002, p. 304. Chang speculates that the melting point of francium would be about 23 °C.
    20. ^ New Scientist 1975; Soverna 2004; Eichler, Aksenov & Belozeroz et al. 2007; Austen 2012
    21. ^ Hunt 2000, p. 256
    22. ^ Sisler 1973, p. 89
    23. ^ Hérold 2006, pp. 149–150
    24. ^ Russell & Lee 2005
    25. ^ Legit, Friák & Šob 2010, p. 214118-18
    26. ^ Manson & Halford 2006, pp. 378, 410
    27. ^ a b McQuarrie & Rock 1987, p. 85
    28. ^ Chung 1987; Godfrin & Lauter 1995
    29. ^ Cambridge Enterprise 2013
    30. ^ Faraday 1853, p. 42; Holderness & Berry 1979, p. 255
    31. ^ Partington 1944, p. 405
    32. ^ Regnault 1853, p. 208
    33. ^ Christensen 2012, p. 14
    34. ^ Gschneidner 1964, pp. 292‒93.
    35. ^ Qin et al. 2012, p. 258
    36. ^ Hopcroft, Nix & Kenny 2010, p. 236
    37. ^ Greaves et al. 2011, p. 826
    38. ^ Brassington et al. 1980
    39. ^ Martienssen & Warlimont 2005, p. 100
    40. ^ Witczak 2000, p. 823
    41. ^ Marlowe 1970, p. 6;Slyh 1955, p. 146
    42. ^ Klein & Cardinale 1992, pp. 184‒85
    43. ^ Appalakondaiah et al. 2012, pp. 035105‒6
    44. ^ Sundara Rao 1950; Sundara Rao 1954; Ravindran 1998, pp. 4897‒98
    45. ^ Lindegaard & Dahle 1966, p. 264
    46. ^ Leith 1966, pp. 38‒39
    47. ^ Donohoe 1982; Russell & Lee 2005
    48. ^ Gupta et al. 2005, p. 502
    49. ^ Walker, Newman & Enache 2013, p. 25
    50. ^ Wiberg 2001, p. 143
    51. ^ Batsanov & Batsanov 2012, p. 275
    52. ^ Clementi & Raimondi 1963; Clementi, Raimondi & Reinhardt 1967
    53. ^ Addison 1964; Donohoe 1982
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    References

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