Post-transition metal

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. The most common name, post-transition metals, is generally used in this article.
Physically, these metals are soft, 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. They can also form Zintl phases.

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.
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. 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 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. 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 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 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', 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."

Descriptive chemistry

Group 10

Platinum is a moderately hard metal of low mechanical strength, with a close-packed face-centred cubic structure. Compared to other metals in this category, it has an unusually high melting point. 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 coordination complexes. The oxide is amphoteric, with acidic properties predominating; it can be fused with alkali hydroxides or calcium oxide to give anionic platinates, such as red Na₂PtO₃ and green K₂PtO₃. The hydrated oxide can be dissolved in hydrochloric acid to give the hexachlormetallate, 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 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 oxide should be amphoteric, and darmstadtium oxide 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." 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 with low mechanical strength. It has a close-packed face-centred cubic structure. 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 have significant covalent character. The oxide is amphoteric, with predominating basic properties; it can be fused with alkali oxides to give anionic oxycuprates. Copper forms Zintl phases such as Li₇CuSi₂ and M₃Cu₃Sb₄.
Silver is a soft metal with low mechanical strength. It has a close-packed face-centred cubic structure. 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 is amphoteric, with basic properties predominating. Silver forms a series of oxoargentates. It is a constituent of Zintl phases such as Li₂AgM and Yb₃Ag₂.
Gold is a soft metal that is easily deformed. It has a close-packed face-centred cubic structure. 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 is amphoteric, with acidic properties predominating; it forms anionic hydroxoaurates, where M = Na, K, ½Ba, Tl; and aurates such as NaAuO₂. Gold is a constituent of Zintl phases such as M₂AuBi ; Li₂AuM 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 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, 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. 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.
Zinc is a soft metal with poor mechanical properties. It has a crystalline structure 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 Zn₂, 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 that undergoes substantial deformation, under load, at room temperature. Like zinc, it has a crystalline structure 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 Cd₂, 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, 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 and melting point which, together, are the lowest of all the metallic elements. Solid mercury has a distorted crystalline structure, with mixed metallic-covalent bonding, and a BCN of 6. "All of the metals, but especially mercury, tend to form covalent rather than ionic compounds." The oxide of mercury in its preferred oxidation state is weakly amphoteric, as is the congener sulfide HgS. It forms anionic thiomercurates in strongly basic solutions. 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. 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 is expected to be predominantly basic.