Interrelationships between the Lightest Metals

Abstract

As defined here, the “lightest” metals are those pretransition metals of lowest atomic number; that is, lithium, beryllium, sodium, magnesium, aluminum, potassium, and calcium (atomic no. 3–20); they have relatively low densities (0.53–2.70 g cm −3 ), but are not a set of the least dense metals. This article considers various attributes that link these metals together, starting with their primordial nucleosynthesis, and the problem of the cosmic abundance of lithium. It is believed that the relatively low natural abundance of both lithium and beryllium on the Earth is ultimately a consequence of peculiarities in their formation. Apart from these two, however, the “lightest” metals are extremely common elements on the Earth: Al is the most abundant metal in the crust (over 8% by weight), and the metals Ca, Mg, Na, and K constitute the fifth through eighth most abundant elements in the crust. It is perhaps not surprising that their ions (Ca 2+ , Mg 2+ , Na + , and K + ) are the most encountered metal species in biological systems. Although long used in the form of their compounds, the metals were not isolated as the free elements until the early nineteenth century (the last produced was beryllium, in 1828). They have been recognized as sharing similar chemical properties since the time they were discovered, leading to the articulation of the “(iso)diagonal rule”, which highlights the similarities between lithium and magnesium compounds, and between those of beryllium and aluminum. The metals' mechanical properties vary considerably; the stiffness of beryllium (it possesses the smallest Poisson ratio of any bulk element) leads to its supporting the highest velocity of sound of any metal; in contrast, lithium, sodium, and potassium are so soft that many of their elastic properties are difficult to measure accurately. Various alloys of the metals have extremely important properties, from the aluminum–magnesium alloys used in the aerospace industry to the beryllium copper alloys used to make nonsparking tools. Essential to life and indisputably important to modern society, the range of applications of “the lightest metals” can only be expected to grow during the twenty‐first century.