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According to my book,hydrides are classified as Ionic/Saltlike/Saline Hydrides, Covalent/Molecular Hydrides and Metallic/Non-stochiometric/Interstitial Hydrides.

The explanation of my book is not very clear to me I am confused that if an S-block element & Hydrogen form a covalent compound, it will be Saline or Molecular Hydride? Same question for a P-block element which forms ionic bond with Hydrogen.

I will be grateful if you would help.

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    $\begingroup$ Hydrides like $\ce{NaH, CaH2}$ are ionic. Compounds or ions made by hydrogen and non-metals are covalent : $\ce{B2H6, BH4^-, CH4, SiH4, NH3, PH3, AsH3, H2O, OH-, H2S, H2Te, HF, HF2^-, HCl, HBr, HI}$. Even coordinate bonds in $\ce{AlH4^- }$ are covalent. Interstitial hydrides are relatively rare, like $\ce{H2}$ dissolved into palladium. $\endgroup$
    – Maurice
    Commented Sep 20, 2022 at 9:33
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    $\begingroup$ @Maurice - those of us coming from the materials side are more familiar with interstitial hydrides, since that is how most metal hydrides (solid phases) form. Just a matter of perspective... $\endgroup$
    – Jon Custer
    Commented Sep 20, 2022 at 13:40

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Hydrides of alkaline/alkaline earth elements are almost* always ionic because the cations are very electropositive forming a strong ionic bond. Hence, they are called ionic/salt like hydrides

The hydrides of p-block elements are not very strong and are non-ionic, hence they are considered covalent hydrides.

*The key word almost is because beryllium hydride is the only s-block hydride which is covalent. $\ce{BeH2}$ is also the only known s-block hydride which can't be made by direction reaction of beryllium and hydrogen (unlike other s-block hydrides which are made by direct metal and hydrogen interaction)

Previous discussion: Is lithium hydride a salt?

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  • $\begingroup$ But in S block, BeH2 is a covalent compound and in P block, I think GaH3 is an ionic compound. So BeH2 & GaH3 will be Saline/Saltlike or Molecular Hydrides? $\endgroup$
    – Nipun Kulshreshtha
    Commented Sep 20, 2022 at 10:44
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    $\begingroup$ @NipunKulshreshtha I was about to update on BeH2. I will update the answer. But GaH3 is unfortunately covalent. It has been detected as a transient species in the gas phase at very low temperature. However, it is stable in its dimeric form of formula $\ce{H2Ga(H2)GaH2}$ which points towards its covalency and polymerism. $\endgroup$
    – Nilay Ghosh
    Commented Sep 20, 2022 at 10:52
  • $\begingroup$ @NilayGhosh aww come on, what is so unfortunate about gallium hydride being covalent? $\endgroup$
    – Oscar Lanzi
    Commented Oct 16, 2022 at 21:39
  • $\begingroup$ @OscarLanzi OP assumed gallium hydride was ionic/salt like :) $\endgroup$
    – Nilay Ghosh
    Commented Oct 18, 2022 at 7:50

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