Hydrides — Core Principles

Core Principles

Hydrides are binary compounds formed between hydrogen and other elements. Their classification depends on the nature of the bond, which is primarily determined by the electronegativity difference between hydrogen and the other element. There are three main types:

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Ionic (Saline) Hydrides: — Formed by Group 1 and heavier Group 2 metals. Hydrogen exists as $H^-$ (hydride ion). They are solid, non-volatile, conduct electricity in molten state, and react vigorously with water to produce $H_2$ gas. Examples: $NaH$, $CaH_2$.

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Covalent (Molecular) Hydrides: — Formed by p-block elements and some s-block (Be, Mg). Hydrogen forms covalent bonds. They are generally volatile (gases/liquids) and non-conductors. Sub-classified into electron-deficient (e.g., $B_2H_6$), electron-precise (e.g., $CH_4$), and electron-rich (e.g., $NH_3$, $H_2O$, $HF$). Electron-rich hydrides exhibit hydrogen bonding.

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Metallic (Interstitial) Hydrides: — Formed by many d-block and f-block elements. Hydrogen atoms occupy interstitial sites in the metal lattice. They are often non-stoichiometric (e.g., $TiH_{1.7}$), retain metallic conductivity, and are important for hydrogen storage. Group 7, 8, 9 elements do not form hydrides (hydride gap).

Important Differences

vs Ionic, Covalent, and Metallic Hydrides

Aspect	This Topic	Ionic, Covalent, and Metallic Hydrides
Elements Involved	Group 1 & heavier Group 2 metals (e.g., Na, Ca)	p-block elements (Gr 13-17) & some s-block (Be, Mg)
Bonding Type	Ionic (electrostatic attraction between $M^+$ and $H^-$)	Covalent (sharing of electron pairs)
Physical State (at RT)	Crystalline solids	Gases or liquids (some solids for heavier elements)
Electrical Conductivity	Insulators in solid state; conductors in molten state	Non-conductors
Stoichiometry	Stoichiometric (fixed integer ratio, e.g., NaH, CaH$_2$)	Stoichiometric (fixed integer ratio, e.g., CH$_4$, H$_2$O)
Reactivity with Water	Vigorous reaction, produces $H_2$ gas and metal hydroxide	Generally stable, some react (e.g., $B_2H_6$ with $H_2O$)
Reducing Nature	Strong reducing agents (due to $H^-$)	Variable (some are reducing, some are not)

The classification of hydrides into ionic, covalent, and metallic types is fundamental to understanding their diverse chemical and physical properties. Ionic hydrides, formed by highly electropositive metals, are salt-like solids with $H^-$ ions, exhibiting high melting points and vigorous reactivity with water. Covalent hydrides, formed by p-block elements, are volatile molecular compounds with shared electron bonds, further categorized by electron count. Metallic hydrides, formed by transition metals, are often non-stoichiometric solids where hydrogen occupies interstitial sites, retaining metallic conductivity and showing promise for hydrogen storage.