Wikipedia:Reference desk/Archives/Science/2021 August 23

= August 23 =

What are the densest known compounds without metallic bonds?
Not no metal, just no metallic bonds. Sagittarian Milky Way (talk) 14:02, 23 August 2021 (UTC)
 * Neutronium. -- Jayron 32 16:05, 23 August 2021 (UTC)
 * And what about compounds of elements made of electrons, protons and (except for hydrogen-1) neutrons, measured at standard temperature and pressure? Sagittarian Milky Way (talk) 17:12, 23 August 2021 (UTC)
 * This article mentions basalt in passing, but that has a density of about 3 g/cm3. That's not that impressive.  This discussion at Quora cites tungsten carbide and uranium carbide as much higher in density, around 13-14 g/cm3, which is getting pretty impressive.  Bonding happens on a continuum, so the qualification "without metallic bonds" is a bit tricky to define around the edges, and these both have some metallic character.  -- Jayron 32 17:21, 23 August 2021 (UTC)
 * I was looking at osmium and iridium compounds with better atom count ratios and only got up to 11.66 (IrO2), tungsten carbide is much better (15.63 according to the article. So apparently I want to look at dense metal atomides of nonmetals close to the gap size and formulas around 1:1. Sagittarian Milky Way (talk) 18:01, 23 August 2021 (UTC)
 * The reasons why metals are so dense is twofold. The first is that metallic bonding (the "sea-of-electrons") allows for close packing of the atoms in a crystal lattice.  Secondly, and most important for our purposes, is that atoms are spheres, which can be packed MUCH closer than other sorts of compounds, for example I would expect no molecular compounds to come anywhere near metallic densities, molecules are not spheres and don't pack efficiently.  The exceptions seem to be the metallic-carbide and metallic-sulfide compounds, which seem relatively small and seem to pack fairly close.  -- Jayron 32 18:06, 23 August 2021 (UTC)
 * You might argue that the carbides are metallic in character. If the substance is transparent or white there will be no metallic nature. And so we have Lutetium tantalate with 9.8, the densest stable white material. If things are put under pressure they will become more dense, but will spring back when pressure is released. Thorium dioxide is white and has a higher density of 10, and if colour is allowed plutonium(IV) oxide has 11.5 and americium dioxide with 11.68. but they are radioactive and are hard to get. Graeme Bartlett (talk) 22:52, 26 August 2021 (UTC)
 * So for no metallic character at all and stable it is a matrix of 2 metals with oxygens to keep them apart, clever. Sagittarian Milky Way (talk) 23:46, 26 August 2021 (UTC)