Hydrides Classification
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Direct answer
Binary hydrides sort by bond type into three working families plus a reagent shelf. Saline (ionic) hydrides of groups 1 and 2 — NaH, CaH2 — hold the H− ion, melt high, conduct in the molten state and hydrolyse briskly to hydrogen. Covalent (molecular) hydrides of the p-block, from CH4 through NH3, H2O and HF, are volatile discrete molecules, subdivided by electron count into electron-deficient (diborane), electron-precise (group 14) and electron-rich (ammonia, water, hydrogen fluoride with lone pairs). Metallic (interstitial) hydrides absorb hydrogen into d-block lattices as non-stoichiometric phases such as TiH1.73, and complex hydrides LiAlH4 and NaBH4 serve as the organic chemist's reducing agents.
What you must remember
- Saline hydrides: NaH crystallises in the rock-salt arrangement; CaH2 + 2H2O → Ca(OH)2 + 2H2 makes calcium hydride a drying agent; thermal stability of group 1 hydrides falls down the group, LiH the most stable.
- Boundary cases: BeH2 and MgH2 are polymeric and covalent, not ionic — small polarising cations pull the bonding towards covalency (Fajans logic).
- Electron-deficient hydrides: diborane's bridges are three-centre-two-electron bonds, making B2H6 a Lewis acid; it hydrolyses as B2H6 + 6H2O → 2B(OH)3 + 6H2.
- Electron-precise and electron-rich: group 14 hydrides use every electron in σ bonds; NH3, H2O and HF keep lone pairs — Lewis bases and hydrogen-bond donors, which is why ammonia boils far above phosphine.
- Metallic/interstitial: non-stoichiometric conducting phases such as TiH1.73, VH0.56 and LaH2.87; the hydride gap — metals of groups 7-9 around Mn, Fe, Co and Ni form few or no hydrides.
- Applications: LaNi5 absorbs about six hydrogens per formula unit — the nickel-metal-hydride battery and hydrogen-storage logic.
- Complex hydrides: LiAlH4 reduces most carbonyl compounds; NaBH4 is its mild, alcohol-solvent counterpart.
- Water-reactivity ladder: saline hydrides react violently; silane and diborane hydrolyse; ammonia dissolves basic; methane stays inert — behaviour tracks bond polarity.
Sorting an unknown hydride
Classification questions hand you clues, and each clue votes. A crystalline solid with a high melting point that conducts when molten and releases flammable gas on water is saline — think CaH2 drying a solvent. A volatile liquid or gas of a p-block element is covalent; then count electrons: diborane cannot satisfy conventional two-centre bonds, so it bridges (electron-deficient), methane spends everything on σ bonds (precise), and ammonia banks a lone pair (rich, hence basic and hydrogen-bonded — the fact behind its anomalous boiling point). A metal powder that soaks up hydrogen reversibly at moderate pressure is an interstitial hydride; if the metal is iron or nickel, the expected answer is the hydride gap instead. A white crystalline solid used to reduce aldehydes in the laboratory is LiAlH4, a complex hydride that behaves ionic toward the counter-ion yet covalent through its bridging Al–H bonds. Sorting, in every case, is bond-type recognition wearing different clothes.
The borderline cases examiners probe
The hydrogen chapter left the JEE Main syllabus in the 2023-24 rationalisation, but hydride logic has not left the ecosystem: JEE Advanced retains hydrogen-family chemistry in its revised syllabus, LiAlH4 and NaBH4 appear every year inside organic reductions, and saline hydrides resurface in hydrogen-yield numericals (CaH2 with water is a classic). The examined edges are the boundaries. BeH2 and MgH2 wrongly filed as ionic by candidates who see a group 2 metal. Hydride-gap metals wrongly grouped with titanium and zirconium as absorbers. The electron-rich trio's boiling points — NH3 above PH3, H2O above H2S — presented as hydride facts that are really hydrogen-bond facts. And the double identity of H− as both strong base and strong reductant, which is why ionic hydrides both hydrolyse and reduce. Hold the boundary list and this page converts from memory work to reasoning.
Frequently asked questions
Name one hydride each of the saline, covalent and metallic types.
NaH (ionic H− in a rock-salt lattice), CH4 or NH3 (discrete covalent molecules), and TiH1.73 (non-stoichiometric interstitial phase).
Why are group 13 hydrides electron-deficient?
Their valence electrons cannot fill conventional two-centre two-electron bonds, forcing bridged multi-centre bonding as in diborane's B–H–B links.
Which metals resist hydride formation?
The group 7-9 cluster around manganese, iron, cobalt and nickel — the hydride gap — absorbs little hydrogen, unlike early transition metals or palladium.
Why does calcium hydride serve as a drying agent?
It reacts stoichiometrically with traces of water, CaH2 + 2H2O → Ca(OH)2 + 2H2, stripping moisture from solvents and gases.
What role do complex hydrides play in synthesis?
LiAlH4 and NaBH4 deliver hydride to electrophilic carbon, reducing aldehydes, ketones, acids and esters to alcohols with different vigour and solvent tolerance.