Post-transition metal

Post-transition metals in the periodic table   Elements classified as post-transition metals by Masterton, Hurley and Neth:[1] Ga, In, Tl, Sn, Pb, Bi, Po   Also recognised by Huheey, Keiter and Keiter:[2] Al, Ge, Sb, Po; and by Cox:[3] Zn, Cd, Hg   Also recognised by Deming:[4] Cu, Ag, Au (but he counted Al and groups 1 and 2 as 'light metals')[n 1]   Elements that might be post-transition metals: At, Cn, Nh, Fl, Mc, Lv, Ts

In chemistry, post-transition metals are the metallic elements in the periodic table located between the transition metals (to their left) and the metalloids (to their right). Usually included in this category are gallium, indium and thallium; tin and lead; and bismuth. Which elements are counted as post-transition metals depends, in periodic table terms, on where the transition metals are taken to end and where the metalloids or non-metals are taken to start.

Physically, post-transition metals are soft (or brittle), have poor mechanical strength, and melting points lower than those of the transition metals; most also have boiling points lower than those of the transition metals. Being close to the metal-nonmetal border, their crystalline structures tend to show covalent or directional bonding effects, having generally greater complexity or fewer nearest neighbours than other metallic elements.

Chemically, they are characterised—to varying degrees—by covalent bonding tendencies, acid-base amphoterism and the formation of anionic species such as aluminates, stannates, and bismuthates (in the case of aluminium, tin, and bismuth, respectively). They can also form Zintl phases (half-metallic compounds formed between highly electropositive metals and moderately electronegative metals or metalloids).

The expression 'post-transition metals' is used here as there is no IUPAC-approved collective name for these metals. Occasionally, some or all of them have instead been referred to as B-subgroup metals, other metals, or p-block metals; and by at least eleven other alternative labels. All these labels are surveyed later in this article.

Applicable elements

Scatter plot of electronegativity values and melting points for metals (up to fermium, element 100) and some borderline elements (Ge, As, Sb, At). Elements categorised by some authors as post-transition metals are distinguished by their relatively high electronegativity values and relatively low melting points. High electronegativity corresponds to increasing nonmetallic character;^[8] low melting temperature corresponds to weaker cohesive forces between atoms and reduced mechanical strength.^[9] The geography of the plot broadly matches that of the periodic table. Starting from the bottom left, and proceeding clockwise, the alkali metals are followed by the heavier alkaline earth metals; the rare earths and actinides (Sc, Y and the lanthanides being here treated as rare earths); transition metals with intermediate electronegativity values and melting points; the refractory metals; the platinum group metals; and the coinage metals leading and forming a part of the post-transition metals. The increased electropositivity of Be and Mg and the higher melting point of Be distances these light alkaline earth metals from their heavier congeners. This separation extends to other differences in physical and chemical behaviour between the light and heavier alkaline earth metals.^{[n 2]}

Usually included in this category are the group 13–15 metals: gallium, indium and thallium; tin and lead; and bismuth. Other elements sometimes included are copper, silver and gold (which are usually considered to be transition metals); zinc, cadmium and mercury (which are otherwise considered to be transition metals); and aluminium, germanium, arsenic and antimony (the latter three of which are usually considered to be metalloids). Astatine, which is usually classified as a nonmetal or a metalloid, has been predicted to have a metallic crystalline structure. If so, it would be a post-transition metal. Elements 112–117 may be post-transition metals; insufficient quantities of them have been synthesized to allow investigation of their actual physical and chemical properties.

Which elements start to be counted as post-transition metals depends, in periodic table terms, on where the transition metals are taken to end.^{[n 3]} In the 1950s, most inorganic chemistry textbooks defined transition elements as finishing at group 10 (nickel, palladium and platinum), therefore excluding group 11 (copper, silver and gold), and group 12 (zinc, cadmium and mercury). A survey of chemistry books in 2003 showed that the transition metals ended at either group 11 or group 12 with roughly equal frequency.^[13] Where the post-transition metals end depends on where the metalloids or nonmetals start. Boron, silicon, germanium, arsenic, antimony and tellurium are commonly recognised as metalloids; other authors treat some or all of these elements as nonmetals.

Rationale

The diminished metallic nature of the post-transition metals is largely attributable to the increase in nuclear charge going across the periodic table, from left to right.^[14] The increase in nuclear charge is partially offset by an increasing number of electrons but as these are spatially distributed each extra electron does not fully screen each successive increase in nuclear charge, and the latter therefore dominates.^[15] With some irregularities, atomic radii contract, ionisation energies increase,^[14] fewer electrons become available for metallic bonding,^[16] and "ions [become] smaller and more polarizing and more prone to covalency."^[17] This phenomenon is more evident in period 4–6 post-transition metals, due to inefficient screening of their nuclear charges by their d¹⁰ and (in the case of the period 6 metals) f¹⁴ electron configurations;^[18] the screening power of electrons decreases in the sequence s > p > d > f. The reductions in atomic size due to the interjection of the d- and f-blocks are referred to as, respectively, the 'scandide' or 'd-block contraction',^{[n 4]} and the 'lanthanide contraction'.^[19] Relativistic effects also "increase the binding energy", and hence ionisation energy, of the electrons in "the 6s shell in gold and mercury, and the 6p shell in subsequent elements of period 6."^[20]

Nomenclature

Origin

The origin of the term post-transition metal is unclear. An early usage is recorded by Deming, in 1940, in his well-known^[21] book Fundamental Chemistry.^[4] He treated the transition metals as finishing at group 10 (nickel, palladium and platinum). He referred to the ensuing elements in periods 4 to 6 of the periodic table (copper to germanium; silver to antimony; gold to polonium)—in view of their underlying d¹⁰ electronic configurations—as post-transition metals.

Descriptive chemistry

This section outlines relevant physical and chemical properties of the elements typically or sometimes classified as post-transition metals. For complete profiles, including history, production, specific uses, and biological roles and precautions, see the main article for each element. Abbreviations: MH—Mohs hardness; BCN—bulk coordination number.^{[n 5]}

Group 11

The group 11 metals are typically categorised as transition metals given they can form ions with incomplete d-shells. Physically, they have the relatively low melting points and high electronegativity values associated with post-transition metals. "The filled d subshell and free s electron of Cu, Ag, and Au contribute to their high electrical and thermal conductivity. Transition metals to the left of group 11 experience interactions between s electrons and the partially filled d subshell that lower electron mobility."^[24] Chemically, the group 11 metals in their +1 valence states show similarities to other post-transition metals;^[25] they are occasionally classified as such.^[26]

Copper

Silver

Gold

Copper is a soft metal (MH 2.5–3.0)^[27] with low mechanical strength.^[28] It has a close-packed face-centred cubic structure (BCN 12).^[29] Copper behaves like a transition metal in its preferred oxidation state of +2. Stable compounds in which copper is in its less preferred oxidation state of +1 (Cu₂O, CuCl, CuBr, CuI and CuCN, for example) have significant covalent character.^[30] The oxide (CuO) is amphoteric, with predominating basic properties; it can be fused with alkali oxides (M₂O; M = Na, K) to give anionic oxycuprates (M₂CuO₂).^[31] Copper forms Zintl phases such as Li₇CuSi₂^[32] and M₃Cu₃Sb₄ (M = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, or Er).^[33]

Silver is a soft metal (MH 2.5–3)^[34] with low mechanical strength.^[35] It has a close-packed face-centred cubic structure (BCN 12).^[36] The chemistry of silver is dominated by its +1 valence state in which it shows generally similar physical and chemical properties to compounds of thallium, a main group metal, in the same oxidation state.^[37] It tends to bond covalently in most of its compounds.^[38] The oxide (Ag₂O) is amphoteric, with basic properties predominating.^[39] Silver forms a series of oxoargentates (M₃AgO₂, M = Na, K, Rb).^[40] It is a constituent of Zintl phases such as Li₂AgM (M = Al, Ga, In, Tl, Si, Ge, Sn or Pb)^[41] and Yb₃Ag₂.^[42]

Gold is a soft metal (MH 2.5–3)^[43] that is easily deformed.^[44] It has a close-packed face-centred cubic structure (BCN 12).^[36] The chemistry of gold is dominated by its +3 valence state; all such compounds of gold feature covalent bonding,^[45] as do its stable +1 compounds.^[46] Gold oxide (Au₂O₃) is amphoteric, with acid properties predominating; it forms anionic hydroxoaurates M[Au(OH)₄] where M = Na, K,  ¹⁄₂Ba, Tl; and aurates such as NaAuO₂.^[47] Gold is a constituent of Zintl phases such as M₂AuBi (M = Li or Na);^[48] Li₂AuM (M = In, Tl, Ge, Pb, Sn)^[49] and Ca₅Au₄.^[42]

Group 12

On the group 12 transition metals (zinc, cadmium and mercury), Smith^[50] observed that, "Textbook writers have always found difficulty in dealing with these elements." There is an abrupt and significant reduction in physical metallic character from group 11 to group 12.^[51] Their chemistry is that of main group elements.^[52] A 2003 survey of chemistry books showed that they were treated as either transition metals or main group elements on about a 50/50 basis.^{[13][n 6]} The IUPAC Red Book notes that although the group 3−12 elements are commonly referred to as the transition elements, the group 12 elements are not always included.^[54] The group 12 elements do not satisfy the IUPAC Gold Book definition of a transition metal.^{[55][n 7]}

Zinc

Cadmium

Mercury

Zinc is a soft metal (MH 2.5) with poor mechanical properties.^[57] It has a crystalline structure (BCN 6+6) that is slightly distorted from the ideal. Many zinc compounds are markedly covalent in character.^[58] The oxides of zinc in its preferred oxidation state of +2, namely ZnO and Zn(OH)₂, are amphoteric;^[59] it forms anionic zincates in strongly basic solutions.^[60] Zinc forms Zintl phases such as LiZn, NaZn₁₃ and BaZn₁₃.^[61] Highly purified zinc, at room temperature, is ductile.^[62] It reacts with moist air to form a thin layer of carbonate that prevents further corrosion.^[63]

Cadmium is a soft, ductile metal (MH 2.0) that undergoes substantial deformation, under load, at room temperature.^[64] Like zinc, it has a crystalline structure (BCN 6+6) that is slightly distorted from the ideal. The halides of cadmium, with the exception of the fluoride, exhibit a substantially covalent nature.^[65] The oxides of cadmium in its preferred oxidation state of +2, namely CdO and Cd(OH)₂, are weakly amphoteric; it forms cadmates in strongly basic solutions.^[66] Cadmium forms Zintl phases such as LiCd, RbCd₁₃ and CsCd₁₃.^[61] When heated in air to a few hundred degrees, cadmium represents a toxicity hazard due to the release of cadmium vapour; when heated to its boiling point in air (just above 1000 K; 725 C; 1340 F; cf steel ~2700 K; 2425 C; 4400 F),^[67] the cadmium vapour oxidizes, 'with a reddish-yellow flame, dispersing as an aerosol of potentially lethal CdO particles.'^[64] Cadmium is otherwise stable in air and in water, at ambient conditions, protected by a layer of cadmium oxide.

Mercury is a liquid at room temperature. It has the weakest metallic bonding of all, as indicated by its bonding energy (61 kJ/mol) and melting point (−39 °C) which, together, are the lowest of all the metallic elements.^{[68][n 8]} Solid mercury (MH 1.5)^[69] has a distorted crystalline structure,^[70] with mixed metallic-covalent bonding,^[71] and a BCN of 6. "All of the [Group 12] metals, but especially mercury, tend to form covalent rather than ionic compounds."^[72] The oxide of mercury in its preferred oxidation state (HgO; +2) is weakly amphoteric, as is the congener sulfide HgS.^[73] It forms anionic thiomercurates (such as Na₂HgS₂ and BaHgS₃) in strongly basic solutions.^{[74][n 9]} It forms or is a part of Zintl phases such as NaHg and K₈In₁₀Hg.^[75] Mercury is a relatively inert metal, showing little oxide formation at room temperature.^[76]

Group 13

Aluminium

Gallium

Indium

Thallium

Aluminium sometimes is^[77] or is not^[3] counted as a post-transition metal. It has a well shielded [Ne] noble gas core rather than the less well shielded [Ar]3d¹⁰, [Kr]4d¹⁰ or [Xe]4f¹⁴5d¹⁰ core of the post-transition metals. The small radius of the aluminium ion combined with its high charge make it a strongly polarizing species, prone to covalency.^[78]

Aluminium in pure form is a soft metal (MH 3.0) with low mechanical strength.^[79] It has a close-packed structure (BCN 12) showing some evidence of partially directional bonding.^{[80][n 10]} It has a low melting point and a high thermal conductivity. Its strength is halved at 200 °C, and for many of its alloys is minimal at 300 °C.^[82] The latter three properties of aluminium limit its use to situations where fire protection is not required,^[83] or necessitate the provision of increased fire protection.^{[84][n 11]} It bonds covalently in most of its compounds;^[88] has an amphoteric oxide; and can form anionic aluminates.^[60] Aluminium forms Zintl phases such as LiAl, Ca₃Al₂Sb₆, and SrAl₂.^[89] A thin protective layer of oxide confers a reasonable degree of corrosion resistance.^[90] It is susceptible to attack in low pH (<4) and high (> 8.5) pH conditions,^{[91][n 12]} a phenomenon that is generally more pronounced in the case of commercial purity aluminium and aluminium alloys.^[97] Given many of these properties and its proximity to the dividing line between metals and nonmetals, aluminium is occasionally classified as a metalloid.^{[n 13]} Despite its shortcomings, it has a good strength-to-weight ratio and excellent ductility; its mechanical strength can be improved considerably with the use of alloying additives; its very high thermal conductivity can be put to good use in heat sinks and heat exchangers;^[98] and it has a high electrical conductivity.^{[n 14]} At lower temperatures, aluminium increases its deformation strength (as do most materials) whilst maintaining ductility (as do face-centred cubic metals generally).^[100] Chemically, bulk aluminium is a strongly electropositive metal, with a high negative electrode potential.^{[101][n 15]}

Gallium is a soft, brittle metal (MH 1.5) that melts at only a few degrees above room temperature.^[103] It has an unusual crystalline structure featuring mixed metallic-covalent bonding and low symmetry^[103] (BCN 7 i.e. 1+2+2+2).^[104] It bonds covalently in most of its compounds,^[105] has an amphoteric oxide;^[106] and can form anionic gallates.^[60] Gallium forms Zintl phases such as Li₂Ga₇, K₃Ga₁₃ and YbGa₂.^[107] It is slowly oxidized in moist air at ambient conditions; a protective film of oxide prevents further corrosion.^[108]

Indium is a soft, highly ductile metal (MH 1.0) with a low tensile strength.^[109][110] It has a partially distorted crystalline structure (BCN 4+8) associated with incompletely ionised atoms.^[111] The tendency of indium '...to form covalent compounds is one of the more important properties influencing its electrochemical behavior'.^[112] The oxides of indium in its preferred oxidation state of +3, namely In₂O₃ and In(OH)₃ are weakly amphoteric; it forms anionic indates in strongly basic solutions.^[113] Indium forms Zintl phases such as LiIn, Na₂In and Rb₂In₃.^[114] Indium does not oxidize in air at ambient conditions.^[110]

Thallium is a soft, reactive metal (MH 1.0), so much so that it has no structural uses.^[115] It has a close-packed crystalline structure (BCN 6+6) but an abnormally large interatomic distance that has been attributed to partial ionisation of the thallium atoms.^[116] Although compounds in the +1 (mostly ionic) oxidation state are the more numerous, thallium has an appreciable chemistry in the +3 (largely covalent) oxidation state, as seen in its chalcogenides and trihalides.^[117] It is the only one of the Group 13 elements to react with air at room temperature, slowly forming the amphoteric oxide Tl₂O₃.^{[118][119][120]} It forms anionic thallates such as Tl₃TlO₃, Na₃Tl(OH)₆, NaTlO₂, and KTlO₂,^[119] and is present as the Tl⁻ thallide anion in the compound CsTl.^[121] Thallium forms Zintl phases, such as Na₂Tl, Na₂K₂₁Tl₁₉, CsTl and Sr₅Tl₃H.^[122]

Group 14

Germanium

Tin

Lead

Germanium is a hard (MH 6), very brittle semi-metallic element.^[123] It was originally thought to be a poorly conducting metal^[124] but has the electronic band structure of a semiconductor.^[125] Germanium is usually considered to be a metalloid rather than a metal.^[126] Like carbon (as diamond) and silicon, it has a covalent tetrahedral crystalline structure (BCN 4).^[127] Compounds in its preferred oxidation state of +4 are covalent.^[128] Germanium forms an amphoteric oxide, GeO₂^[129] and anionic germanates, such as Mg₂GeO₄.^[130] It forms Zintl phases such as LiGe, K₈Ge₄₄ and La₄Ge₃.^[131]

Tin is a soft, exceptionally^[132] weak metal (MH 1.5);^{[n 16]} a 1-cm thick rod will bend easily under mild finger pressure.^[132] It has an irregularly coordinated crystalline structure (BCN 4+2) associated with incompletely ionised atoms.^[111] All of the Group 14 elements form compounds in which they are in the +4, predominantly covalent, oxidation state; even in the +2 oxidation state tin generally forms covalent bonds.^[134] The oxides of tin in its preferred oxidation state of +2, namely SnO and Sn(OH)₂, are amphoteric;^[135] it forms stannites in strongly basic solutions.^[60] Below 13 °C (55.4 °F) tin changes its structure and becomes 'grey tin', which has the same structure as diamond, silicon and germanium (BCN 4). This transformation causes ordinary tin to crumble and disintegrate since, as well as being brittle, grey tin occupies more volume due to having a less efficient crystalline packing structure. Tin forms Zintl phases such as Na₄Sn, BaSn, K₈Sn₂₅ and Ca₃₁Sn₂₀.^[136] It has good corrosion resistance in air on account of forming a thin protective oxide layer. Pure tin has no structural uses.^[137] It is used in lead-free solders, and as a hardening agent in alloys of other metals, such as copper, lead, titanium and zinc.^[138]

Lead is a soft metal (MH 1.5)(but hardens close to melting) which, in many cases,^[139] is unable to support its own weight.^[140] It has a close-packed structure (BCN 12) but an abnormally large inter-atomic distance that has been attributed to partial ionisation of the lead atoms.^[116][141] It forms a semi-covalent dioxide PbO₂; a covalently bonded sulfide PbS; covalently bonded halides;^[142] and a range of covalently bonded organolead compounds such as the lead(II) mercaptan Pb(SC₂H₅)₂, lead tetra-acetate Pb(CH₃CO₂)₄, and the once common, anti-knock additive, tetra-ethyl lead (CH₃CH₂)₄Pb.^[143] The oxide of lead in its preferred oxidation state (PbO; +2) is amphoteric;^[144] it forms anionic plumbates in strongly basic solutions.^[60] Lead forms Zintl phases such as CsPb, Sr₃₁Pb₂₀, La₅Pb₃N and Yb₃Pb₂₀.^[145] It has reasonable to good corrosion resistance; in moist air it forms a mixed gray coating of oxide, carbonate and sulfate that hinders further oxidation.^[146]

Group 15

Arsenic

Antimony

Bismuth

Arsenic is a moderately hard (MH 3.5) and brittle semi-metallic element. It is commonly regarded as a metalloid, or by some other authors as either a metal or a non-metal. It exhibits poor electrical conductivity which, like a metal, decreases with temperature. It has a relatively open and partially covalent crystalline structure (BCN 3+3). Arsenic forms covalent bonds with most other elements. The oxide in its preferred oxidation state (As₂O₃, +3) is amphoteric,^{[n 17]} as is the corresponding oxoacid in aqueous solution (H₃AsO₃) and congener sulphide (As₂S₃). Arsenic forms a series of anionic arsenates such as Na₃AsO₃ and PbHAsO₄, and Zintl phases such as Na₃As, Ca₂As and SrAs₃.

Antimony is a soft (MH 3.0) and brittle semi-metallic element. It is commonly regarded as a metalloid, or by some other authors as either a metal or a non-metal. It exhibits poor electrical conductivity which, like a metal, decreases with temperature. It has a relatively open and partially covalent crystalline structure (BCN 3+3). Antimony forms covalent bonds with most other elements. The oxide in its preferred oxidation state (Sb₂O₃, +3) is amphoteric. Antimony forms a series of anionic antimonites and antimonates such as NaSbO₂ and AlSbO₄, and Zintl phases such as K₅Sb₄, Sr₂Sb₃ and BaSb₃.

Bismuth is a soft metal (MH 2.5) that is too brittle for any structural use.^[149] It has an open-packed crystalline structure (BCN 3+3) with bonding that is intermediate between metallic and covalent.^[150] For a metal, it has exceptionally low electrical and thermal conductivity.^[151] Most of the ordinary compounds of bismuth are covalent in nature.^[152] The oxide, Bi₂O₃ is predominantly basic but will act as a weak acid in warm, very concentrated KOH.^[153] It can also be fused with potassium hydroxide in air, resulting in a brown mass of potassium bismuthate.^[154] The solution chemistry of bismuth is characterised by the formation of oxyanions;^[155] it forms anionic bismuthates in strongly basic solutions.^[156] Bismuth forms Zintl phases such as NaBi,^[157] Rb₇In₄Bi₆^[158] and Ba₁₁Cd₈Bi₁₄.^[159] Bailar et al.^[160] refer to bismuth as being, 'the least "metallic" metal in its physical properties' given its brittle nature (and possibly) 'the lowest electrical conductivity of all metals.'^{[n 18]}

Group 16

Polonium is a radioactive, soft metal with a hardness similar to lead.^[162] It has a simple cubic crystalline structure characterised (as determined by electron density calculations) by partially directional bonding,^[163] and a BCN of 6. Such a structure ordinarily results in very low ductility and fracture resistance^[164] however polonium has been predicted to be a ductile metal.^[165] It forms a covalent hydride;^[166] its halides are covalent, volatile compounds, resembling those of tellurium.^[167] The oxide of polonium in its preferred oxidation state (PoO₂; +4) is predominantly basic, but amphoteric if dissolved in concentrated aqueous alkali, or fused with potassium hydroxide in air.^[168] The yellow polonate(IV) ion PoO2−
3 is known in aqueous solutions of low Cl^‒ concentration and high pH.^{[169][n 19]} Polonides such as Na₂Po, BePo, ZnPo, CdPo and HgPo feature Po²⁻ anions;^[171] except for HgPo these are some of the more stable of the polonium compounds.^{[172][n 20]}

Group 17

Astatine is a radioactive element that has never been seen; a visible quantity would immediately be vapourized due to its intense radioactivity.^[174] It may be possible to prevent this with sufficient cooling.^[175] Astatine is commonly regarded as a nonmetal,^[176] less commonly as a metalloid^[177] and occasionally as a metal. Unlike its lighter congener iodine, evidence for diatomic astatine is sparse and inconclusive.^[178] In 2013, on the basis of relativistic modelling, astatine was predicted to be a monatomic metal, with a face-centered cubic crystalline structure.^[175] As such, astatine could be expected to have a metallic appearance; show metallic conductivity; and have excellent ductility, even at cryogenic temperatures.^[179] It could also be expected to show significant nonmetallic character, as is normally the case for metals in, or in the vicinity of, the p-block. Astatine oxyanions AtO⁻, AtO−
2, AtO−
3 and AtO−
4 are known,^[180] oxyanion formation being a tendency of nonmetals.^[181] The hydroxide of astatine At(OH) is presumed to be amphoteric.^{[182][n 21]} Astatine forms covalent compounds with nonmetals,^[185] including hydrogen astatide HAt and carbon tetraastaide CAt₄.^{[186][n 22]} At⁻ anions have been reported to form astatides with silver, thallium, palladium and lead.^[188] Pruszyński et al. note that astatide ions should form strong complexes with soft metal cations such as Hg²⁺, Pd²⁺, Ag⁺ and Tl³⁺; they list the astatide formed with mercury as Hg(OH)At.^[189]

Related groupings

B-subgroup metals

Superficially, the B-subgroup metals are the metals in Groups IB to VIB of the periodic table, corresponding to Groups 11 to 16 using current IUPAC nonmenclature. Practically, the group 11 metals (copper, silver and gold) are ordinarily regarded as transition metals (or sometimes as coinage metals, or noble metals) whereas the group 12 metals (zinc, cadmium, and mercury) may or may not be treated as B-subgroup metals depending on if the transition metals are taken to end at group 11 or group 12. The 'B' nomenclature (as in Groups IB, IIB, and so on) was superseded in 1988 but is still occasionally encountered in more recent literature.^{[190][n 23]}

The B-subgroup metals show nonmetallic properties; this is particularly apparent in moving from group 12 to group 16.^[192] Although the group 11 metals have normal close-packed metallic structures^[193] they show an overlap in chemical properties. In their +1 compounds (the stable state for silver; less so for copper)^[194] they are typical B-subgroup metals. In their +2 and +3 states their chemistry is typical of transition metal compounds.^[195]

Borderline metals

Parish^[196] writes that, 'as anticipated', the borderline metals of groups 13 and 14 have non-standard structures. Gallium, indium, thallium, germanium, and tin are specifically mentioned in this context. The group 12 metals are also noted as having slightly distorted structures; this has been interpreted as evidence of weak directional (i.e. covalent) bonding.^{[n 24]}

Chemically weak metals

Rayner-Canham and Overton^[198] use the term chemically weak metals to refer to the metals close to the metal-nonmetal borderline. These metals behave chemically more like the metalloids, particularly with respect to anionic species formation. The nine chemically weak metals identified by them are beryllium, aluminium, zinc, gallium, tin, lead, antimony, bismuth, and polonium.^{[n 25]}

Fusible metals

Cardarelli,^[200] writing in 2008, categorizes zinc, cadmium, mercury, gallium, indium, thallium, tin, lead, antimony and bismuth as fusible metals. Nearly 100 years earlier, Louis (1911)^[201] noted that fusible metals were alloys containing tin, cadmium, lead, and bismuth in various proportions, "the tin ranging from 10 to 20%."

Heavy metals (of low melting point)

Van Wert^[202] grouped the periodic table metals into a. the light metals; b. the heavy brittle metals of high melting point, c. the heavy ductile metals of high melting point; d. the heavy metals of low melting point (Zn, Cd, Hg; Ga, In, Tl; Ge, Sn; As, Sb, Bi; and Po), and e. the strong, electropositive metals. Britton, Abbatiello and Robins^[203] speak of 'the soft, low melting point, heavy metals in columns lIB, IlIA, IVA, and VA of the periodic table, namely Zn, Cd, Hg; Al, Ga, In, Tl; [Si], Ge, Sn, Pb; and Bi. The Sargent-Welch Chart of the Elements groups the metals into: light metals, the lanthanide series; the actinide series; heavy metals (brittle); heavy metals (ductile); and heavy metals (low melting point): Zn, Cd, Hg, [Cn]; Al, Ga, In, Tl; Ge, Sn, Pb, [Fl]; Sb, Bi; and Po.^{[204][n 26]}

Less typical metals

Habashi^[206] groups the elements into eight major categories: [1] typical metals (alkali metals, alkaline earth metals, and aluminium); [2] lanthanides (Ce–Lu); [3] actinides (Th–Lr); [4] transition metals (Sc, Y, La, Ac, groups 4–10); [5] less typical metals (groups 11–12, Ga, In, Tl, Sn and Pb); [6] metalloids (B, Si, Ge, As, Se, Sb, Te, Bi and Po); [7] covalent nonmetals (H, C, N, O, P, S and the halogens); and [8] monatomic nonmetals (that is, the noble gases).

Metametals

The metametals are zinc, cadmium, mercury, indium, thallium, tin and lead. They are ductile elements but, compared to their metallic periodic table neighbours to the left, have lower melting points, relatively low electrical and thermal conductivities, and show distortions from close-packed forms.^[207] Sometimes beryllium^[208] and gallium^[209] are included as metametals despite having low ductility.

Ordinary metals

Abrikosov^[210] distinguishes between ordinary metals, and transition metals where the inner shells are not filled. The ordinary metals have lower melting points and cohesive energies than those of the transition metals.^[211] Gray^[212] identifies as ordinary metals: aluminium, gallium, indium, thallium, nihonium, tin, lead, flerovium, bismuth, moscovium, and livermorium. He adds that, 'in reality most of the metals that people think of as ordinary are in fact transition metals...'.

Other metals

As noted, the metals falling between the transition metals and the metalloids on the periodic table are sometimes called other metals (see also, for example, Taylor et al.).^[213] 'Other' in this sense has the related meanings of, 'existing besides, or distinct from, that already mentioned'^[214] (that is, the alkali and alkaline earth metals, the lanthanides and actinides, and the transition metals); 'auxiliary'; 'ancillary, secondary'.^[215] According to Gray^[216] there should be a better name for these elements than 'other metals'.

P-block metals

The p-block metals are the metals in groups 13‒16 of the periodic table. Usually, this includes aluminium, gallium, indium and thallium; tin and lead; and bismuth. Germanium, antimony and polonium are sometimes also included, although the first two are commonly recognised as metalloids. The p-block metals tend to have structures that display low coordination numbers and directional bonding. Pronounced covalency is found in their compounds; the majority of their oxides are amphoteric.^[217]

Peculiar metals

Slater^[218] divides the metals 'fairly definitely, though not perfectly sharply' into the ordinary metals and the peculiar metals, the latter of which verge on the nonmetals. The peculiar metals occur towards the ends of the rows of the periodic table and include 'approximately:' gallium, indium, and thallium; carbon, silicon '(both of which have some metallic properties, though we have previously treated them as nonmetals),' germanium and tin; arsenic, antimony, and bismuth; and selenium '(which is partly metallic)' and tellurium. The ordinary metals have centro-symmetrical crystalline structures^{[n 27]} whereas the peculiar metals have structures involving directional bonding. More recently, Joshua observed that the peculiar metals have mixed metallic-covalent bonding.^[220]

Poor metals

Farrell and Van Sicien^[221] use the term poor metal, for simplicity, 'to denote one with a significant covalent, or directional character.' Hill and Holman^[222] observe that, 'The term poor metals is not widely used, but it is a useful description for several metals including tin, lead and bismuth. These metals fall in a triangular block of the periodic table to the right of the transition metals. They are usually low in the activity (electrochemical) series and they have some resemblances to non-metals.' Reid et al.^[223] write that 'poor metals' is, '[A]n older term for metallic elements in Groups 13‒15 of the periodic table that are softer and have lower melting points than the metals traditionally used for tools.'

Semimetals

In modern use, the term 'semimetal' sometimes refers, loosely or explicitly, to metals with incomplete metallic character in crystalline structure, electrical conductivity or electronic structure. Examples include gallium,^[224] ytterbium,^[225] bismuth,^[226] mercury^[227] and neptunium.^[228] Metalloids, which are in-between elements that are neither metals nor nonmetals, are also sometimes instead called semimetals. The elements commonly recognised as metalloids are boron, silicon, germanium, arsenic, antimony and tellurium. In old chemistry, before the publication in 1789 of Lavoisier's 'revolutionary'^[229] Elementary Treatise on Chemistry,^[230] a semimetal was a metallic element with 'very imperfect ductility and malleability'^[231] such as zinc, mercury or bismuth.

Transition metals

Historically, the transition metal series "includes those elements of the Periodic Table which 'bridge the gap' between the very electropositive alkali and allkaline earth metals and the electronegative non-metals of the groups: nitrogen-phosphorus, oxygen-sulfur, and the halogens."^[232] Cheronis, Parsons and Ronneberg^[233] wrote that, "The transition metals of low melting point form a block in the Periodic Table: those of Groups II 'b' [zinc, cadmium, mercury], III 'b' [aluminium, gallium, indium, thallium], and germanium, tin and lead in Group IV. These metals all have melting points below 425 °C."^{[n 28]}

Notes

1. More recent examples of authors who treat Cu, Ag and Au as post-transition metals include Subba Rao & Shafer;^[5] Collings;^[6] and Temkin.^[7]
2. Physical properties: "The lighter alkaline earths possess fairly high electrical and thermal conductivities and sufficient strength for structural use. The heavier elements are poor conductors and are too weak and reactive for structural use."^[10] Chemical: The lighter alkaline earths show covalent bonding tendencies (Be predominantly; Mg considerably) whereas compounds of the heavier alkaline earths are predominantly ionic in nature; the heavier alkaline earths have more stable hydrides and less stable carbides.^[11]
3. A first IUPAC definition states "[T]he elements of groups 3–12 are the d-block elements. These elements are also commonly referred to as the transition elements, though the elements of group 12 are not always included". Depending on the inclusion of group 12 as transition metals, the post-transition metals therefore may or may not include the group 12 elements—zinc, cadmium, and mercury. A second IUPAC definition for transition metals states "An element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Based on this definition one could argue group 12 should be split with mercury and probably also copernicium as transition metals, and zinc and cadmium as post-transition metals. Of relevance is the synthesis of mercury(IV) fluoride, which seemingly establishes mercury as a transition metal. This conclusion has been challenged by Jensen^[12] with the argument that HgF₄ only exists under highly atypical non-equilibrium conditions (at 4 K) and should best be considered as an exception. Copernicium has been predicted to have (a) an electron configuration similar to that of mercury; and (b) a predominance of its chemistry in the +4 state, and on that basis would be regarded as a transition metal. However, in recent years, doubt has been cast on the synthesis of HgF₄ and the possible existence of copernicium(IV), so that group 12 would have only post-transition metals.
4. The scandide contraction refers to the first row transition metals; the d-block contraction is a more general term.
5. Moh's hardness values are taken from Samsanov,^[22] unless otherwise noted; bulk coordination number values are taken from Darken and Gurry,^[23] unless otherwise noted.
6. The group 12 metals have been treated as transition metals for reasons of historical precedent, to compare and contrast properties, to preserve symmetry, or for basic teaching purposes.^[53]
7. The IUPAC Gold Book defines a transition metal as 'An element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell.^[56]
8. Francium may have a comparably low bonding energy but its melting point of around 27°C is significantly higher than that of mercury, at −39°C.
9. Mercury also forms partially anionic oxomercurates, such as Li₂HgO₂ and CdHgO₄, by heating mixtures of HgO with the relevant cation oxides, including under oxygen pressure (Müller-Buschbaum 1995; Deiseroth 2004, pp. 173, 177, 185–186).
10. The partially directional bonding in aluminium improves its shear strength but means that ultrahigh-purity aluminium cannot maintain work hardening at room temperature.^[81]
11. Without the use of thermal insulation and detailed structural design attention,^[85] aluminium's low melting point and high thermal conductivity mitigate against its use, for example, in military ship construction—should a ship burn, the low melting point results in structural collapse; the high thermal conductivity helps spread the fire.^[86] Its use in the construction of cargo ships is limited as little or no economic advantage is gained over steel, once the cost and weight of fitting thermal insulation is taken into account.^[87]
12. Aluminium can be attacked, for example, by alkaline detergents^[92] (including those used in dishwashers);^[93] by wet concrete,^[94] and by highly acidic foods such as tomatoes, rhubarb or cabbage.^[95] It is not attacked by nitric acid.^[96]
13. See the list of metalloid lists for references
14. Aluminium wire is used in electrical transmission lines for the distribution of power but, on account of its low breaking strength, is refinforced with a central core of galvanised steel wire.^[99]
15. In the absence of protective measures, the relatively high electropositivity of aluminium renders it susceptible to galvanic corrosion when in physical or electrical contact with other metals such as copper or steel, especially when exposed to saline media, such as sea water or wind-blown sea spray.^[102]
16. Charles, Crane and Furness write that, 'Most metals, except perhaps lead and tin, can be alloyed to give [yield] strengths that lie in the upper two-thirds of the low-strength range…'^[133]
17. As₂O₃ is usually regarded as being amphoteric but a few sources say it is (weakly)^[147] acidic. They describe its "basic" properties (its reaction with concentrated hydrochloric acid to form arsenic trichloride) as being alcoholic, in analogy with the formation of covalent alkyl chlorides by covalent alcohols (e.g., R-OH + HCl → RCl + H₂O)^[148]
18. Which metal has the lowest electrical conductivity is debatable but bismuth is certainly in the lowest cohort; Hoffman^[161] refers to bismuth as 'a poor metal, on the verge of being a semiconductor.'
19. Bagnall^[170] writes that the fusion of polonium dioxide with a potassium chlorate/hydroxide mixture yields a bluish solid which, '...presumably contains some potassium polonate.'
20. Bagnall^[173] noted that the rare-earth polonides have the greatest thermal stability of any polonium compound.
21. Eagleson refers to the OH compound of astatine as hypoastatous acid HAtO;^[183] Pimpentel and Spratley give the formula for hypoastatous acid as HOAt.^[184]
22. In hydrogen astatide the negative charge is predicted to be on the hydrogen atom,^[187] implying that this compound should instead be referred to as astatine hydride (AtH).
23. Greenwood and Earnshaw^[191] refer to the B-subgroup metals as post-transition elements: 'Arsenic and antimony are classed as metalloids or semi-metals and bismuth is a typical B sub-group (post-transition-element) metal like tin and lead.'
24. Aluminium is identified by Parish, along with germanium, antimony and bismuth, as being a metal on the boundary line between metals and non-metals; he suggests that all these elements are 'probably better classed as metalloids.'^[197]
25. Pauling,^[199] in contrast, refers to the strong metals in Groups 1 and 2 (that form ionic compounds with 'the strong nonmetals in the upper right corner of the periodic table.').
26. Hawkes,^[205] attempting to address the question of what is a heavy metal, commented that, 'Being a heavy metal has little to do with density, but rather concerns chemical properties'. He observed that, 'It may mean different things to different people, but as I have used, heard and interpreted the term over the last half-century, it refers to metals with insoluble sulfides and hydroxides, whose salts produce colored solutions in water, and whose complexes are usually colored.' He goes on to note that, 'The metals I have seen referred to as heavy metals comprise a block of all the metals in Groups 3 to 16 that are in periods 4 and greater. It may also be stated as the transition metals and post-transition metals.
27. On manganese, Slater says, '[It] is a very peculiar and anomalous exception to the general order of the elements. It is the only definite metal, far from the nonmetals in the table, which has a complicated structure.'^[219]
28. In fact, both aluminium (660.32) and germanium (938.25) have melting points greater than 425°C.

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