Hydride (compounds of hydrogen with other elements)

Types and bonding

Hydrides are commonly classified by the nature of the bond between hydrogen and the other element. The principal categories are:

• Ionic (saline) hydrides — formed when electropositive metals transfer electron density to hydrogen, producing species often described as M+ and H−. These solids are usually reactive with water and can be difficult to dissolve; for notes on solubility see solubility considerations. Many of the alkali metals and alkaline earth metals give ionic hydrides.
• Interstitial (metallic) hydrides — hydrogen atoms occupy interstitial sites in a metal lattice without forming discrete molecules. These materials retain metallic conduction and heat transport; see general information on electrical conductivity and thermal conductivity. They are typical of certain transition and early d-block metals (commonly grouped among groups 3–5 metals).
• Covalent hydrides — hydrogen shares electrons with nonmetals to form molecules. Most p‑block elements form covalent hydrides; examples include hydrocarbons (hydrides of carbon), ammonia (a hydride of nitrogen) and water (the hydride of oxygen).

Formation and occurrence

Hydrides are prepared or encountered in many ways: direct combination of elements under appropriate conditions, reduction reactions that add hydrogen to an element or molecule, or by insertion of hydrogen into a metal lattice at elevated temperatures and pressures. In nature, covalent hydrides such as water and organic molecules are ubiquitous. Ionic hydrides are commonly formed in controlled laboratory or industrial settings when very electropositive metals react with hydrogen or hydrogen donors.

Uses and importance

Different hydride types have distinct technological and scientific roles. Examples include:

• Hydrogen storage and transport: some interstitial metal hydrides can reversibly absorb and release hydrogen and are studied for energy storage.
• Battery materials: metal hydrides are used in rechargeable battery electrodes, notably in nickel–metal hydride cells where an alloy stores hydrogen.
• Chemical synthesis: ionic and covalent hydride reagents (for example, hydride donors) are essential in reductions and organic transformations.
• Analytical and laboratory uses: hydride generation techniques and hydride-forming reactions are used in trace element analysis and gas-phase studies.

Reactivity and safety

Reactivity varies widely. Ionic hydrides are often strongly basic and react violently with water to release hydrogen gas, whereas many covalent hydrides are flammable or decomposed by heat and oxygen. Interstitial hydrides can change the mechanical and electronic properties of metals and sometimes embrittle alloys. Safe handling practices depend on the hydride type; for example, preventing moisture contact with ionic hydrides and avoiding uncontrolled heating of flammable hydride gases.

Distinctions and notable facts

Beyond the simple categories, chemists also distinguish hydrides by electronic character (nucleophilic "hydride" H−, protic H+ in acids, and radical H·) and by stoichiometry and structure. The field spans fundamental chemistry to applied materials science and energy research. For more detailed technical resources and reviews consult specialized literature or introductory pages such as those referenced above (definition, electrical, periodic). This article gives a concise orientation; readers seeking synthetic procedures, thermodynamic data, or industrial practices should consult authoritative sources dedicated to those topics.