The metallic elements in the periodic table located between the transition metals to their left and the chemically weak nonmetallic metalloids to their right have received many names in the literature, such as post-transition metals, poor metals, other metals, p-block metals, basic metals, and chemically weak metals.
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).
Applicable elements
The post-transition metals are located on the periodic table between the transition metals to their left and the chemically weak nonmetallic metalloids or nonmetals to their right. Generally included in this category are: the group 13–16 metals in periods 4–6 namely gallium, indium and thallium, tin and lead, bismuth, and polonium; and aluminium, a group 13 metal in period 3.
They can be seen at the bottom right in the accompanying plot of electronegativity values and melting points.
The boundaries of the category are not necessarily sharp as there is some overlapping of properties with adjacent categories (as occurs with classification schemes generally).Jones 2010, pp. 169–71
Some elements otherwise counted as transition metals are sometimes instead counted as post-transition metals namely the group 10 metal platinum; the group 11 coinage metals copper, silver and gold; and, more often, the group 12 metals zinc, cadmium and mercury.
Similarly, some elements otherwise counted as metalloids or nonmetals are sometimes instead counted as post-transition metals namely 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 through oganesson) may be post-transition metals; insufficient quantities of them have been synthesized to allow sufficient investigation of their actual physical and chemical properties.
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.Cox 2004, p. 17 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.Atkins & de Paula 2011, p. 352 With some irregularities, atomic radii contract, ionisation energies increase, fewer electrons become available for metallic bonding,Greenwood & Earnshaw 1998, pp. 222–3 and "ions [become] smaller and more polarizing and more prone to covalency."Steele 1966, p. 193 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;Johnson 1970 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', and the 'lanthanide contraction'.Huheey & Huheey 1972, p. 229; Mason 1988 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."Cox 2004, pp. 20, 186, 188
Descriptive chemistry
Group 10
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.Reith & Shushter 2018, p. 115 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;Van Loon & Barefoot 1991, p. 52 most platinum compounds are (covalent) coordination complexes.Pauling 1988, p. 695 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.Lidin 1996, p. 347; Wiberg, Holleman & Wiberg 2001, p. 1521
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.Karpov, Konuma & Jansen M 2006, p. 839
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.
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."Russell & Lee 2005, p. 302 Chemically, the group 11 metals in their +1 valence states show similarities to other post-transition metals;Steele 1966, p. 67 they are occasionally classified as such.Deming 1940, pp. 705–7; Karamad, Tripkovic & Rossmeisl 2014
Copper is a soft metal (MH 2.5–3.0)Cheemalapati, Keleher & Li 2008, p. 226 with low mechanical strength.Liu & Pecht 2004, p. 54 It has a close-packed face-centred cubic structure (BCN 12).Donohue 1982, p. 222 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.Vanderah 1992, p. 52 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).Lidin 1996, p. 110 Copper forms Zintl phases such as Li7CuSi2Slabon et al. 2012 and M3Cu3Sb4 (M = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, or Er).Larson et al. 2006, p. 035111-2
Silver is a soft metal (MH 2.5–3)Schumann 2008, p. 52 with low mechanical strength.Braunović 2014, p. 244 It has a close-packed face-centred cubic structure (BCN 12).Donohue 1982, p. 222 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.Banthorpe, Gatforde & Hollebone 1968, p. 61; Dillard & Goldberg 1971, p. 558 It tends to bond covalently in most of its compounds.Steiner & Campbell 1955, p. 394 The oxide (Ag2O) is amphoteric, with basic properties predominating.Lidin 1996, p. 5 Silver forms a series of oxoargentates (M3AgO2, M = Na, K, Rb).Klassen & Hoppe 1982; Darriet, Devalette & Lecart 1977; Sofin et al. 2002 It is a constituent of Zintl phases such as Li2AgM (M = Al, Ga, In, Tl, Si, Ge, Sn or Pb)Goodwin et al. 2005, p. 341 and Yb3Ag2.Köhler & Whangbo 2008
Gold is a soft metal (MH 2.5–3)Arndt & Ganino 2012, p. 115 that is easily deformed.Goffer 2007, p. 176 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,Sidgwick 1950, p. 177 as do its stable +1 compounds.Pauling 1988, p. 698 Gold oxide (Au2O3) is amphoteric, with acidic properties predominating; it forms anionic hydroxoaurates , where M = Na, K, ½Ba, Tl; and aurates such as NaAuO2.Lidin 1996, p. 21–22 Gold is a constituent of Zintl phases such as M2AuBi (M = Li or Na);Miller et al. 2011, p. 150 Li2AuM (M = In, Tl, Ge, Pb, Sn)Fishcher-Bünher 2011, p. 150 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 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.
Group 12
On the group 12 metals (zinc, cadmium and mercury), SmithSmith 1990, p. 113 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.Sorensen 1991, p. 3 Their chemistry is that of main group elements.King 1995, pp. xiii, 273–288; Cotton et al. 1999, pp. ix, 598; Massey 2000, pp. 159–176 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.Jensen 2003, p. 952 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.IUPAC 2005, p. 51 The group 12 elements do not satisfy the IUPAC Gold Book definition of a transition metal.Crichton 2012, p. 11