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Hydrogen is generally considered a non-metal, and a non-metal and a metal often makes a salt. However, lithium hydride seems like a special case. The wikipedia article does not clearly state if the compound is a salt or not, only describing it as "salt-like".

Structurally, it makes sense to classify it as a salt, due to the network structure, but are there any clear definition to settle where the compound belongs?

LiH

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  • $\begingroup$ Many metals form hydrides; what's so special about lithium? $\endgroup$ – Ivan Neretin Feb 18 '16 at 9:40
  • $\begingroup$ @IvanNeretin Can hydrides be considered salts? $\endgroup$ – Hohmannfan Feb 18 '16 at 9:42
  • $\begingroup$ Salt doesn't mean ionic compound. And there's no purely ionic or covalent compound. $\endgroup$ – Mithoron Feb 18 '16 at 18:29
  • $\begingroup$ Ah! A commonly used classification may be the best way to handle this then $\endgroup$ – Hohmannfan Feb 18 '16 at 21:21
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The Wikipedia article Salt (chemistry) starts off with "In chemistry, a salt is an ionic compound that results from the neutralization reaction of an acid and a base." This seems like a pretty good Chemistry 101 definition. So, in essence, this means having an anion and a cation in the salt, which further implies ionic bonding. This seems like a pretty reasonable place to start.

I'd think that the definition could be tweaked to say "a salt is a solid ionic compound..." That would indicate that one "molecule" isn't a salt. So one molecule of NaCl in the gas phase is a molecule, not a salt. I also have trouble thinking of molten NaCl as a salt. So I'd describe it as a "molten salt."

Kind of like way I think German works. I'd expect that German would have some compound word for "molten salt" which better denotes that there is one new idea. So in English if a "salt" is a solid that has a 3D crystal lattice how can a salt be "molten"?

In general, there are a lot more organic salts than inorganic, so dissolving in water isn't a good restriction.

These last paragraphs point out the sorts of problem in trying to make such a definition absolutely unambiguous. You get a one sentence definition, then two pages of qualifications which render the definition useless.

I can't think of a solvent in which you could dissolve lithium hydride and I don't see how you could get lithium hydride as a neutralization reaction, so I wouldn't think of it as a salt. I don't know a better word than "salt-like" to describe such a crystal structure of anions and cations.

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  • $\begingroup$ According to your new definition, wouldn't sodium oxide also be a salt? $\endgroup$ – Pritt Balagopal Jun 26 '17 at 14:45
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Hydrides of metallic elements are ionic compounds. In the case of $\ce{LiH}$ there may be some covalent characters due to the small size of the $\ce{Li+}$ ion. This hydride on electrolysis in molten form gives $\ce{H2}$ on the anode, which is an indication of the presence of hydride ions and, further, this hydride reaction with water or protic acid evolves $\ce{H2}$ gas.

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    $\begingroup$ This answer is a bit vague. For instance, metal carbonyl hydrides often have particularly non-hydridic "hydride" ligands. When mixed with water, they may form $H_2$, but not before acting as a Lewis acid to form a more electron-rich metal complex (by displacing the carbonyl ligands with water/hydroxide ligands). $\endgroup$ – SendersReagent Jan 15 '17 at 18:59

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