p-block nuance and subtlety

[Rene]

This item was prompted by:

• Jess’ recent post about liquid carbon (thanks Jess).
• Our group’s ongoing interest in the occurrence of symmetry and regularity.
• What happens to the metallic or nonmetallic character of the solid p-block elements when they’re heated with a view to melting them.

The scope is the top-right 5 × 5 portion of the p-block, spanning periods 2 to 6 across groups 13 to 17. Figure 1 attached shows the post-melting-attempt status of these elements, indicating whether they retain or change their metallic or nonmetallic nature—or whether they sublime instead of melting.

Melting, and exceptions
Of the elements that melt, most preserve their metallic or nonmetallic identity. But there are noteworthy exceptions:

• Silicon and germanium, both nonmetals in the solid state, become liquid metals.
• Arsenic, though a semimetal in the physics-based sense, sublimes at ambient pressure, forming a nonmetallic molecular vapor (As4 and As2).
• Tellurium initially retains its semiconducting (nonmetallic) character just above its melting point (723 K), but undergoes a transition to metallic conductivity from around 850–1000 K.

Other elements of interest:

• Carbon and phosphorus, like arsenic, sublime rather than melt under standard conditions.
• Iodine, while well-known for subliming, can be melted under carefully controlled conditions.

The special case of astatine
Though astatine is predicted to be metallic due to relativistic spin–orbit and dispersion effects, its metallic character may not survive heating. The bonding in solid astatine is likely too weak and too sensitive to thermal expansion and orbital decoherence to sustain a robust metallic liquid. As with arsenic, this suggests modest cohesive energy and a readiness to sublime, even from a metallic lattice. Thus, while metallic in its ground state, astatine may vaporise into a mixture of atomic and weakly bound oligomeric species, behaving more like a volatile metalloid than a classic metal.

Symmetry and broken symmetry
In the image, the thick black dividing line between metals and nonmetals is symmetrical, with its axis running through arsenic. In ambient conditions, this symmetry is preserved—except for relativistic effects, which are expected to result in astatine being a metal. By following the thick black line, the four thick red lines show how the conventional stairstep line is also symmetrical, again with arsenic at its centre.

A–B diagonal: B–Si–As–Te–At
This line exhibits broken symmetry. The alternating sequence is almost nonmetal → metal → nonmetal → metal → nonmetal, with solid arsenic subliming to serve as the pivot point. I say “almost” because tellurium remains semiconducting just after melting (at 723 K), becoming metallic only at somewhat higher temperatures (850–1000 K).

C–D diagonal: Tl–Sn–As–S–F
This diagonal, too, exhibits a striking symmetry.

It again passes through arsenic. Interestingly, the bookends, thallium and fluorine, are both notoriously toxic, as is arsenic itself.

The elements between—sulfur and tin—share some unusual parallels:

Ductility

Sulfur displays metal-like ductility when rapidly quenched from the melt, forming plastic sulfur. Tin, of course, is naturally ductile in its common white form.

Conductivity

Liquid sulfur exhibits an electrical conductivity (5 × 10^−5 S·cm^−1) that is about a trillion times greater than that of solid sulfur at room temperature, and roughly twice that of molten boron, a classic metalloid. Tin approaches nonmetallic behaviour in the form of gray tin, which appears at low temperatures and has a sub-metallic conductivity of (2–5) × 10^2 S·cm^−1—about 1/250 that of white tin.

Oxides

Both SO3 and SnO2 are glass-forming oxides, further blurring the boundary between metallic and nonmetallic behaviour.

Numerology
Arsenic’s atomic number displays bifold symmetry in both decimal (33) and binary (100001) forms. The next such element is einsteinium (99 and 1100011). Fittingly, arsenic sits in row 3, column 3 of the p-block.

Closing thoughts
Nature has done a fine job of weaving nuance and subtlety into the p-block, and the periodic table more broadly—on at least three levels:

• At the centre, arsenic—of all elements—appears to occupy the p-block Grand Central position, with seven mentions in the context of symmetry, pivot points, or centrality.
• Radiating outward, the transition from metallic to nonmetallic character across the p-block—which gives rise to the dividing line between metals and nonmetals—displays a surprising degree of symmetrical structure.
• At the broadest scale, the periodic table itself can be subdivided into four sets of complementary pairs of elements:
• active metals < − >  halogen nonmetals
• mundane transition metals < − >  biocentric nonmetals
• post-transition metals < − >  metalloid nonmetals
• noble metals < − >  noble gases.

Although symmetry and regularity in periodic table, or their absence, have long been discussed in the literature, these three nested levels of order have apparently remained underappreciated until now—seemingly hidden in plain sight.

Have I missed anything?

René