What is it about hydrogen that makes it different from the halides? What makes it only form di- and not tetra- or hexa- hydrides of the chalcogens?
Alkalai Metals bond with Sulfur/Chalcogens in a 2:1 ratio and Hydrogen is behaving like an Alkalai Metal.
Lithium Sulflide = Li2S
Sodium Sulfide = Na2S
Potassium Sulfide = K2S
Rubidium Sulfide = Ru2S
Cesium Sulfide = Ce2S
Francium Sulfide = Fr2S
. . .
Hydrogen Oxide = H2O
Hydrogen Sulfide = H2S
Hydrogen Selenide = H2Se
Hydrogen Telluride = H2Te
If Hydrogen were like a Halogen, it would form H4S, and H6S. However Hydrogen doesn't obey the octet rule like halogens as it only needs to get rid of one electron, and yet it shares 2 electrons in a bond with Sulfur. That is exactly what the alkali metals do: they need to get rid of one electron and they share 2 with sulfur.
Na2S adopts the antifluorite structure, which means that the Na+ centers occupy sites of the fluoride in the CaF2 framework, and the larger S2− occupy the sites for Ca2+. -https://en.wikipedia.org/wiki/Sodium_sulfide
Thus an atoms lacking or exceeding by 2 tends to form 2:1 ratios with atoms lacking or exceeding by 1.
I wonder if He4S and He6S exist or if Helium would bond with Sulfur more like the alalai earth metals that bond to Sulfur in a 1:1 ratio.
Oxygen is really the problematic chalcogen: we are used to the Halogens behaving like hydrogen: See Dioxygen Diflouride and Hydrogen Peroxide. And then Hydrogen goes out and acts like an alkali metal.!