Why H2S2 is hydrogen disulfide and not hydrogen persulfide?
TL;DR: &quot;Hydrogen persulfide&quot; is technically a very crude mixture of various hydrogen polysulfides ($\ce{H2S_n ; n\geq2}$). It has long sulfur chains and is formulated as $\ce{H-(S)_{n}-H}$. To avoid confusion, each members of the mixture is named according to number of sulfur atoms. So, $\ce{H2S2}$ is hydrogen disulfide.
Long answer:
It is prepared by heating sodium sulfide ($\ce{Na2S.9H2O}$) for three hours at 100° C. The crystalline sulfide melts and the sulfur is gradually dissolved to a deep-red solution of which the solute has various kinds of sodium polysulfides. This solution is mixed with rather less than its own bulk of water and is then introduced as a thin stream into a mixture of dilute hydrochloric acid and ice which dissolves the alkali and gives a crude oily mixture which is referred to as &quot;hydrogen persulfide&quot;.
Individual constituent of the crude mixture can also be separated. Treatment of hydrochloric acid to the distillate amounting to approximately one-sixth of the original bulk yields a solution which is called hydrogen trisulfide, $\ce{H2S3}$. A more volatile liquid, hydrogen disulfide, $\ce{H2S2}$, can be condensed in a second receiver cooled by a mixture of ether and solid carbon dioxide.
The crude persulfide can also be formed if a large proportion of sulfur or sulphur dioxide either as gas or in solution is reduced by hypophosphorous acid.
The so-called hydrogen persulfide, are yellow liquids at r.t. They are unreactive towards acids but are very sensitive towards alkalis causing vigorous decomposition. It is stable only for few hours if stored in vessel. Even at r.t., they readily decompose into hydrogen sulfide and sulfur. The compounds burn with a blue flame, giving water and sulfur dioxide. The addition of alcohols induces rapid decomposition producing sulfur. Ketones, nitrobenzene, aniline and pyridine also catalyze the decomposition. It is miscible in benzene, toluene, chloroform, bromoform, carbon disulfide, ether and heptane, giving relatively stable solutions. As such, it finds its use in vulcanization.
Source: http://sulphur.atomistry.com/hydrogen_polysulphides.html