|Poor metals in the periodic table|
|Atomic number color shows state at STP:
In chemistry, the trivial name poor metals is sometimes applied to the metallic elements in the p-block of the periodic table. Their melting and boiling points are generally lower than those of the transition metals and their electronegativity values higher, and they are also softer. Being close to the metal-nonmetal border, their crystalline structures tend to show covalent effects, having generally greater complexity or fewer nearest neighbours than other metallic elements. The poor metals are distinguished from the metalloids by their significantly higher electrical conductivity values and, for elements in the same periodic table row, greater densities.
"Poor metals" is not a rigorous IUPAC-approved term, but the grouping is generally taken to include aluminium, gallium, indium, thallium, tin, lead, bismuth and polonium. Occasionally germanium and antimony are also included, although these are usually considered to be metalloids. Elements 113, 114, 115, and 116, which are currently allocated the names ununtrium, flerovium, ununpentium, and livermorium, would likely exhibit properties characteristic of poor metals; sufficient quantities of them have not yet been synthesized to facilitate an examination of their chemical properties.
The term post-transition metal is generally used to describe the category of metallic elements in periods 4–6 of the periodic table, to the right of the transition elements. Since this description excludes aluminium, a period 3 metal,[n 1] the post-transition elements thereby form a subset of the poor metals.
Which elements are counted as post-transition metals depends, in periodic table terms, on where the transition metals 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. An examination of textbooks and monographs in 2003 revealed that the transition metals ended at either group 11 or group 12 with roughly equal frequency. Occasionally aluminium, germanium or antimony are also included as post-transition metals, although the latter two are usually considered to be metalloids.
- Aluminium is sometimes referred to as a pre-transition metal, along with the group 1 alkali metals and group 2 alkaline earth metals.
- 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 copernicium as transition metals, and zinc and cadmium as post-transition metals. Of relevance is the synthesis of mercury(IV) fluoride, which establishes mercury as a transition metal. Copernicium is predicted to have an electron configuration similar to mercury, predicting it as a transition metal as well.
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