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The metallic elements in the periodic table located between the transition metals to their right and the chemically weak nonmetallic metalloids to their left have received many names in the literature, such as post-transition metals, poor metals, other metals, p-block metals and chemically weak metals. The most common name, post-transition metals, is generally used in this article.
Physically, these metals are soft (or brittle), have poor mechanical strength, and usually have melting 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).
Usually included in this category are the group 13–15 metals in periods 4–6: gallium, indium and thallium; tin and lead; and bismuth. Other elements sometimes included are platinum (usually considered to be a transition metal); the group 11 metals copper, silver and gold (which are usually considered to be transition metals); the group 12 metals zinc, cadmium and mercury (which are otherwise considered to be transition metals); and aluminium, germanium, arsenic, selenium, antimony, tellurium, and polonium (of which germanium, arsenic, antimony, and tellurium 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 112–118 (copernicium, nihonium, flerovium, moscovium, livermorium, tennessine, and oganesson) may be post-transition metals; insufficient quantities of them have been synthesized to allow sufficient 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 2] 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. 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. Arsenic, selenium, and tellurium, though lying to the right of the stairstep line, have occasionally been included as post-transition metals.
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. 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. With some irregularities, atomic radii contract, ionisation energies increase, fewer electrons become available for metallic bonding, and "ions [become] smaller and more polarizing and more prone to covalency." 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; 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 3] and the 'lanthanide contraction'. 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."
Main article: Platinum
Platinum is a moderately hard metal (MH 3.5) of low mechanical strength, with a close-packed face-centred cubic structure (BCN 12). Compared to other metals in this category, it has an unusually high melting point (2042 K v 1338 for gold). Platinum is more ductile than gold, silver or copper, thus being the most ductile of pure metals, but it is less malleable than gold. Like gold, platinum is a chalcophile element in terms of its occurrence in the Earth's crust, preferring to form covalent bonds with sulfur. It behaves like a transition metal in its preferred oxidation states of +2 and +4. There is very little evidence of the existence of simple metal ions in aqueous media; most platinum compounds are (covalent) coordination complexes. The oxide (PtO2) is amphoteric, with acidic properties predominating; it can be fused with alkali hydroxides (MOH; M = Na, K) or calcium oxide (CaO) to give anionic platinates, such as red Na2PtO3 and green K2PtO3. The hydrated oxide can be dissolved in hydrochloric acid to give the hexachlormetallate(IV), H2PtCl6.
Like gold, which can form compounds containing the −1 auride ion, platinum can form compounds containing platinide ions, such as the Zintl phases BaPt, Ba3Pt2 and Ba2Pt, being the first (unambiguous) transition metal to do so.
Darmstadtium should be similar to its lighter homologue platinum. It is expected to have a close-packed body-centered cubic structure. It should be a very dense metal, with a density of 26–27 g/cm3 surpassing all stable elements. Darmstadtium chemistry is expected to be dominated by the +2 and +4 oxidation states, similar to platinum. Darmstadtium(IV) oxide (DsO2) should be amphoteric, and darmstadtium(II) oxide (DsO) basic, exactly analogous to platinum. There should also be a +6 oxidation state, similar to platinum. Darmstadtium should be a very noble metal: the standard reduction potential for the Ds2+/Ds couple is expected to be +1.7 V, more than the +1.52 V for the Au3+/Au couple.
Main article: Group 11 element
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." Chemically, the group 11 metals in their +1 valence states show similarities to other post-transition metals; they are occasionally classified as such.
Copper is a soft metal (MH 2.5–3.0) with low mechanical strength. It has a close-packed face-centred cubic structure (BCN 12). 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. 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). Copper forms Zintl phases such as Li7CuSi2 and M3Cu3Sb4 (M = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, or Er).
Silver is a soft metal (MH 2.5–3) with low mechanical strength. It has a close-packed face-centred cubic structure (BCN 12). 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. It tends to bond covalently in most of its compounds. The oxide (Ag2O) is amphoteric, with basic properties predominating. Silver forms a series of oxoargentates (M3AgO2, M = Na, K, Rb). It is a constituent of Zintl phases such as Li2AgM (M = Al, Ga, In, Tl, Si, Ge, Sn or Pb) and Yb3Ag2.
Gold is a soft metal (MH 2.5–3) that is easily deformed. It has a close-packed face-centred cubic structure (BCN 12). The chemistry of gold is dominated by its +3 valence state; all such compounds of gold feature covalent bonding, as do its stable +1 compounds. Gold oxide (Au2O3) is amphoteric, with acidic properties predominating; it forms anionic hydroxoaurates M[Au(OH)4], where M = Na, K, ½Ba, Tl; and aurates such as NaAuO2. Gold is a constituent of Zintl phases such as M2AuBi (M = Li or Na); Li2AuM (M = In, Tl, Ge, Pb, Sn) and Ca5Au4.
Roentgenium is expected to be similar to its lighter homologue gold in many ways. It is expected to have a close-packed body-centered cubic structure. It should be a very dense metal, with its density of 22–24 g/cm3 being around that of osmium and iridium, the densest stable elements. Roentgenium chemistry is expected to be dominated by the +3 valence state, similarly to gold, in which it should similarly behave as a transition metal. Roentgenium oxide (Rg2O3) should be amphoteric; stable compounds in the −1, +1, and +5 valence states should also exist, exactly analogous to gold. Roentgenium is similarly expected to be a very noble metal: the standard reduction potential for the Rg3+/Rg couple is expected to be +1.9 V, more than the +1.52 V for the Au3+/Au couple. The [Rg(H2O)2]+ cation is expected to be the softest among the metal cations. Due to relativistic stabilisation of the 7s subshell, roentgenium is expected to have a full s-subshell and a partially filled d-subshell, instead of the free s-electron and full d-subshell of copper, silver, and gold.
Main article: Group 12 element
On the group 12 metals (zinc, cadmium and mercury), Smith 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. Their chemistry is that of main group elements. 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.[n 5] 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. The group 12 elements do not satisfy the IUPAC Gold Book definition of a transition metal.[n 6]
Zinc is a soft metal (MH 2.5) with poor mechanical properties. It has a crystalline structure (BCN 6+6) that is slightly distorted from the ideal. Many zinc compounds are markedly covalent in character. The oxide and hydroxide of zinc in its preferred oxidation state of +2, namely ZnO and Zn(OH)2, are amphoteric; it forms anionic zincates in strongly basic solutions. Zinc forms Zintl phases such as LiZn, NaZn13 and BaZn13. Highly purified zinc, at room temperature, is ductile. It reacts with moist air to form a thin layer of carbonate that prevents further corrosion.
Cadmium is a soft, ductile metal (MH 2.0) that undergoes substantial deformation, under load, at room temperature. 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. 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. Cadmium forms Zintl phases such as LiCd, RbCd13 and CsCd13. 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), the cadmium vapour oxidizes, 'with a reddish-yellow flame, dispersing as an aerosol of potentially lethal CdO particles.' 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.[n 7] Solid mercury (MH 1.5) has a distorted crystalline structure, with mixed metallic-covalent bonding, and a BCN of 6. "All of the [Group 12] metals, but especially mercury, tend to form covalent rather than ionic compounds." The oxide of mercury in its preferred oxidation state (HgO; +2) is weakly amphoteric, as is the congener sulfide HgS. It forms anionic thiomercurates (such as Na2HgS2 and BaHgS3) in strongly basic solutions.[n 8] It forms or is a part of Zintl phases such as NaHg and K8In10Hg. Mercury is a relatively inert metal, showing little oxide formation at room temperature.
Copernicium is expected to be a liquid at room temperature, although experiments have so far not succeeded in determining its boiling point with sufficient precision to prove this. Like its lighter congener mercury, many of its singular properties stem from its closed-shell d10s2 electron configuration as well as strong relativistic effects. Its cohesive energy is even less than that of mercury and is likely only higher than that of flerovium. Solid copernicium is expected to crystallise in a close-packed body-centred cubic structure and have a density of about 14.7 g/cm3, decreasing to 14.0 g/cm3 on melting, which is similar to that of mercury (13.534 g/cm3). Copernicium chemistry is expected to be dominated by the +2 oxidation state, in which it would behave like a post-transition metal similar to mercury, although the relativistic stabilisation of the 7s orbitals means that this oxidation state involves giving up 6d rather than 7s electrons. A concurrent relativistic destabilisation of the 6d orbitals should allow higher oxidation states such as +3 and +4 with electronegative ligands, such as the halogens. A very high standard reduction potential of +2.1 V is expected for the Cn2+/Cn couple. In fact, bulk copernicium may even be an insulator with a band gap of 6.4±0.2 V, which would make it similar to the noble gases such as radon, though copernicium has previously been predicted to be a semiconductor or a noble metal instead. Copernicium oxide (CnO) is expected to be predominantly basic.
Main article: Boron group
Aluminium sometimes is or is not 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.
Aluminium in pure form is a soft metal (MH 3.0) with low mechanical strength. It has a close-packed structure (BCN 12) showing some evidence of partially directional bonding.[n 9] 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. The latter three properties of aluminium limit its use to situations where fire protection is not required, or necessitate the provision of increased fire protection.[n 10] It bonds covalently in most of its compounds; has an amphoteric oxide; and can form anionic aluminates. Aluminium forms Zintl phases such as LiAl, Ca3Al2Sb6, and SrAl2. A thin protective layer of oxide confers a reasonable degree of corrosion resistance. It is susceptible to attack in low pH (<4) and high (> 8.5) pH conditions,[n 11] a phenomenon that is generally more pronounced in the case of commercial purity aluminium and aluminium alloys. Given many of these properties and its proximity to the dividing line between metals and nonmetals, aluminium is occasionally classified as a metalloid.[n 12] 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; and it has a high electrical conductivity.[n 13] At lower temperatures, aluminium increases its deformation strength (as do most materials) whilst maintaining ductility (as do face-centred cubic metals generally). Chemically, bulk aluminium is a strongly electropositive metal, with a high negative electrode potential.[n 14]
Gallium is a soft, brittle metal (MH 1.5) that melts at only a few degrees above room temperature. It has an unusual crystalline structure featuring mixed metallic-covalent bonding and low symmetry (BCN 7 i.e. 1+2+2+2). It bonds covalently in most of its compounds, has an amphoteric oxide; and can form anionic gallates. Gallium forms Zintl phases such as Li2Ga7, K3Ga13 and YbGa2. It is slowly oxidized in moist air at ambient conditions; a protective film of oxide prevents further corrosion.
Indium is a soft, highly ductile metal (MH 1.0) with a low tensile strength. It has a partially distorted crystalline structure (BCN 4+8) associated with incompletely ionised atoms. The tendency of indium '...to form covalent compounds is one of the more important properties influencing its electrochemical behavior'. 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. Indium forms Zintl phases such as LiIn, Na2In and Rb2In3. Indium does not oxidize in air at ambient conditions.
Thallium is a soft, reactive metal (MH 1.0), so much so that it has no structural uses. 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. 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. It is the only one of the Group 13 elements to react with air at room temperature, slowly forming the amphoteric oxide Tl2O3. It forms anionic thallates such as Tl3TlO3, Na3Tl(OH)6, NaTlO2, and KTlO2, and is present as the Tl− thallide anion in the compound CsTl. Thallium forms Zintl phases, such as Na2Tl, Na2K21Tl19, CsTl and Sr5Tl3H.
Nihonium is expected to have a hexagonal close-packed crystalline structure, albeit based on extrapolation from those of the lighter group 13 elements: its density is expected to be around 16 g/cm3. A standard electrode potential of +0.6 V is predicted for the Nh+/Nh couple. The relativistic stabilisation of the 7s electrons is very high and hence nihonium should predominantly form the +1 oxidation state; nevertheless, as for copernicium, the +3 oxidation state should be reachable. Because of the shell closure at flerovium caused by spin-orbit coupling, nihonium is also one 7p electron short of a closed shell and would hence form a −1 oxidation state; in both the +1 and −1 oxidation states, nihonium should show more similarities to astatine than thallium. The Nh+ ion is expected to also have some similarities to the Ag+ ion, particularly in its propensity for complexation. Nihonium oxide (Nh2O) is expected to be amphoteric.
Main article: Carbon group
Germanium is a hard (MH 6), very brittle semi-metallic element. It was originally thought to be a poorly conducting metal but has the electronic band structure of a semiconductor. Germanium is usually considered to be a metalloid rather than a metal. Like carbon (as diamond) and silicon, it has a covalent tetrahedral crystalline structure (BCN 4). Compounds in its preferred oxidation state of +4 are covalent. Germanium forms an amphoteric oxide, GeO2 and anionic germanates, such as Mg2GeO4. It forms Zintl phases such as LiGe, K8Ge44 and La4Ge3.
Tin is a soft, exceptionally weak metal (MH 1.5);[n 15] a 1-cm thick rod will bend easily under mild finger pressure. It has an irregularly coordinated crystalline structure (BCN 4+2) associated with incompletely ionised atoms. 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. The oxides of tin in its preferred oxidation state of +2, namely SnO and Sn(OH)2, are amphoteric; it forms stannites in strongly basic solutions. 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. It has good corrosion resistance in air on account of forming a thin protective oxide layer. Pure tin has no structural uses. It is used in lead-free solders, and as a hardening agent in alloys of other metals, such as copper, lead, titanium and zinc.
Lead is a soft metal (MH 1.5, but hardens close to melting) which, in many cases, is unable to support its own weight. 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. It forms a semi-covalent dioxide PbO2; a covalently bonded sulfide PbS; covalently bonded halides; 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. The oxide of lead in its preferred oxidation state (PbO; +2) is amphoteric; it forms anionic plumbates in strongly basic solutions. Lead forms Zintl phases such as CsPb, Sr31Pb20, La5Pb3N and Yb3Pb20. 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.
Flerovium is expected to be a liquid metal due to spin-orbit coupling "tearing" apart the 7p subshell, so that its 7s27p1/22 valence configuration forms a quasi-closed shell similar to those of mercury and copernicium. Solid flerovium should have a face-centered cubic structure and be a rather dense metal, with a density of around 14 g/cm3. Flerovium is expected to have a standard electrode potential of +0.9 V for the Fl2+/Fl couple. Flerovium oxide (FlO) is expected to be amphoteric, forming anionic flerovates in basic solutions.
Main article: Pnictogen
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 16] as is the corresponding oxoacid in aqueous solution (H3AsO3) and congener sulfide (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. It has an open-packed crystalline structure (BCN 3+3) with bonding that is intermediate between metallic and covalent. For a metal, it has exceptionally low electrical and thermal conductivity. Most of the ordinary compounds of bismuth are covalent in nature. The oxide, Bi2O3 is predominantly basic but will act as a weak acid in warm, very concentrated KOH. It can also be fused with potassium hydroxide in air, resulting in a brown mass of potassium bismuthate. The solution chemistry of bismuth is characterised by the formation of oxyanions; it forms anionic bismuthates in strongly basic solutions. Bismuth forms Zintl phases such as NaBi, Rb7In4Bi6 and Ba11Cd8Bi14. Bailar et al. 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 17]
Moscovium is expected to be a quite reactive metal. A standard reduction potential of −1.5 V for the Mc+/Mc couple is expected. This increased reactivity is consistent with the quasi-closed shell of flerovium and the beginning of a new series of elements with the filling of the loosely bound 7p3/2 subshell, and is rather different from the relative nobility of bismuth. Like thallium, moscovium should have a common +1 oxidation state and a less common +3 oxidation state, although their relative stabilities may change depending on the complexing ligands or the degree of hydrolysis. Moscovium(I) oxide (Mc2O) should be quite basic, like that of thallium, while moscovium(III) oxide (Mc2O3) should be amphoteric, like that of bismuth.
Main article: Chalcogen
Selenium is a soft (MH 2.0) and brittle semi-metallic element. It is commonly regarded as a nonmetal, but is sometimes considered a metalloid or even a heavy metal. Selenium has a hexagonal polyatomic (CN 2) crystalline structure. It is a semiconductor with a band gap of 1.7 eV, and a photoconductor meaning its electrical conductivity increases a million-fold when illuminated. Selenium forms covalent bonds with most other elements, noting it can form ionic selenides with highly electropositive metals. The common oxide of selenium (SeO3) is strongly acidic. Selenium forms a series of anionic selenites and selenates such as Na2SeO3, Na2Se2O5, and Na2SeO4, as well as Zintl phases such as Cs4Se16.
Tellurium is a soft (MH 2.25) and brittle semi-metallic element. It is commonly regarded as a metalloid, or by some authors either as a metal or a non-metal. Tellurium has a polyatomic (CN 2) hexagonal crystalline structure. It is a semiconductor with a band gap of 0.32 to 0.38 eV. Tellurium forms covalent bonds with most other elements, noting it has an extensive organometallic chemistry and that many tellurides can be regarded as metallic alloys. The common oxide of tellurium (TeO2) is amphoteric. Tellurium forms a series of anionic tellurites and tellurates such as Na2TeO3, Na6TeO6, and Rb6Te2O9 (the last containing tetrahedral TeO2−
4 and trigonal bipyramidal TeO4−
5 anions), as well as Zintl phases such as NaTe3.
Polonium is a radioactive, soft metal with a hardness similar to lead. It has a simple cubic crystalline structure characterised (as determined by electron density calculations) by partially directional bonding, and a BCN of 6. Such a structure ordinarily results in very low ductility and fracture resistance however polonium has been predicted to be a ductile metal. It forms a covalent hydride; its halides are covalent, volatile compounds, resembling those of tellurium. 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. The yellow polonate(IV) ion PoO2−
3 is known in aqueous solutions of low Cl‒ concentration and high pH.[n 18] Polonides such as Na2Po, BePo, ZnPo, CdPo and HgPo feature Po2− anions; except for HgPo these are some of the more stable of the polonium compounds.[n 19]
Livermorium is expected to be less reactive than moscovium. The standard reduction potential of the Lv2+/Lv couple is expected to be around +0.1 V. It should be most stable in the +2 oxidation state; the 7p3/2 electrons are expected to be so weakly bound that the first two ionisation potentials of livermorium should lie between those of the reactive alkaline earth metals magnesium and calcium. The +4 oxidation state should only be reachable with the most electronegative ligands. Livermorium(II) oxide (LvO) should be basic and livermorium(IV) oxide (LvO2) should be amphoteric, analogous to polonium.
Main article: Halogen
Astatine is a radioactive element that has never been seen; a visible quantity would immediately be vaporised due to its intense radioactivity. It may be possible to prevent this with sufficient cooling. Astatine is commonly regarded as a nonmetal, less commonly as a metalloid and occasionally as a metal. Unlike its lighter congener iodine, evidence for diatomic astatine is sparse and inconclusive. In 2013, on the basis of relativistic modelling, astatine was predicted to be a monatomic metal, with a face-centered cubic crystalline structure. As such, astatine could be expected to have a metallic appearance; show metallic conductivity; and have excellent ductility, even at cryogenic temperatures. 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−
3 and AtO−
4 are known, oxyanion formation being a tendency of nonmetals. The hydroxide of astatine At(OH) is presumed to be amphoteric.[n 20] Astatine forms covalent compounds with nonmetals, including hydrogen astatide HAt and carbon tetraastatide CAt4.[n 21] At− anions have been reported to form astatides with silver, thallium, palladium and lead. 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.
Tennessine, despite being in the halogen column of the periodic table, is expected to go even further towards metallicity than astatine due to its small electron affinity. The −1 state should not be important for tennessine and its major oxidation states should be +1 and +3, with +3 more stable: Ts3+ is expected to behave similarly to Au3+ in halide media. As such, tennessine oxide (Ts2O3) is expected to be amphoteric, similar to gold oxide and astatine(III) oxide.
Main article: Oganesson
Oganesson is expected to be a very poor "noble gas" and may even be metallised by its large atomic radius and the weak binding of the easily removed 7p3/2 electrons: certainly it is expected to be a quite reactive element that is solid at room temperature and has some similarities to tin, as one effect of the spin–orbit splitting of the 7p subshell is a "partial role reversal" of groups 14 and 18. Due to the immense polarisability of oganesson, it is expected that not only oganesson(II) fluoride but also oganesson(IV) fluoride should be predominantly ionic, involving the formation of Og2+ and Og4+ cations. Oganesson(II) oxide (OgO) and oganesson(IV) oxide (OgO2) are both expected to be amphoteric, similar to the oxides of tin.