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 (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). 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. 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', 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 (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.
Group 11
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![Post-transition metal: 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 (Pt is anomalous in this regard). High electronegativity corresponds to increasing nonmetallic character;[1] low melting temperature corresponds to weaker cohesive forces between atoms and reduced mechanical strength.[2] 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 (the latter three categories are sub-categories of the broader category of transition metals occupying groups 3–12 of the periodic table).
The increased electronegativity 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 1]](https://upload.wikimedia.org/wikipedia/commons/thumb/d/d8/MP_%26_EN_plot_of_metals8c.png/1280px-MP_%26_EN_plot_of_metals8c.png?utm_source=en.wikipedia.org&utm_campaign=parser&utm_content=thumbnail)
