Gallium Arsenide is Germane to Solar Cells

Gallium arsenide, a simple combination of two elements, interconverts light and electricity; GaAs lasers turn electricity into light and GaAs solar panels convert light back into electricity. There are alternative combinations of elements for these tasks, but each has its limits. What strikes me is how gallium and arsenic bookend germanium:

I need a name for "binary combination of elements which brackets and mimics another element." The term isoelectronic is close but doesn't cut it for me. There is a mathematical symmetry about GaAs in view of Ge and it goes like this: (31 + 33)/2 = 32 or, in chemical logic symbols: (Ga + As)/2 = Ge.

Like gallium arsenide, germanium is a photovoltaic material. Google "germanium solar cell" and you will find cutting edge research involving blends of gallium arsenide with germanium. I'm glad there is on-going research into new materials because I am not sure we should be putting arsenic on every rooftop much like we're putting mercury in every lightbulb.

A similar "bookend relation" occurs a couple rows up in the Table between boron, carbon, and nitrogen. Look how boron and nitrogen bracket carbon:

Once again, (5 + 7)/2 = 6. And just like carbon, boron nitride (BN) has both graphite- and diamond-like structures. One type of BN is even harder than diamonds: link

I see a pattern here: the centrality of the carbon group, C, Si, Ge, etc. to the family of main group elements:

Germanium Arrived According To Plan

Germanium is a metalloid and thus only a half-metal. Despite this, it was a "planned" element. By planned I mean that its existence was foretold by the father of the periodic table, Russian chemist Dmitri Mendeleev.

Mendeleev was not the first to bring rhyme and reason to the elements--first came columns--Döbereiner's triads, and later rows--Newlands' Law of Octaves. But Mendeleev properly put together the whole kahuna, or at least what was known at the time. More importantly, he correctly predicted elements. Here's what he knew and what he knew must exist in 1871:

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It's hard to see, but Mendeleev left a blank: "--  = 72 " in Group IV.  He called the unknown element eka-Silicon meaning one below Silicon. Eka-Silcon was discovered in 1886 and had an atomic mass of 72.6 amu. Van der Krogt tells the story of naming Germanium here, along with some Roman history.

Mendeleev also predicted the existence of eka-Aluminum and estimated its mass as 68.  The element was discovered and named Gallium in 1875 and had an atomic mass of 69.7 amu. The agreement between theory and experiment was almost as astonishing as Bode's Law. But Mendeleev's table survived.

That Chicken had a lot of Gall!

Gallium, named after France (or was it a chicken?), lies just west of Germanium in the periodic table. An imaginary (Imaginot?) line separates them. Actually, both Gallium and Germanium are metallic but a clear demarcation runs diagonally east of Germanium, separating the metals from nonmetals--the "haves" and the "have nots" in terms of sharing the electronic wealth:

Once upon a time I briefly worked for an oil company in Cleveland. An older material scientist (whose name escapes me) once showed us interns a neat trick in the lab. He melted some Gallium metal (it melts in your hand) and showed us how a sheet of Aluminum will absorb Gallium (like dissolves like even liquid metals). The older gent, who was a British ex-pat, explained that during WW II, the RAF feared that the Germans would sneak over and sabotage their planes.  In those days, aircraft were often made of unpainted Aluminum.

Here is a video showing how Gallium wrecks an aluminum coke can. Imagine some nefarious kraut doing that to an airplane's wings.

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Actually, Aluminum is not absorbing--rather, Gallium is invading.

The reason that Gallium liquifies is related to why Copper is so ductile and bears repeating: if you give an atom a perfectly filled sub shell along with one extra electron (the 31^st one), the metallic bonding will be weak and non-directional. It all makes perfect sense to me.