A hydride is any compound in which hydrogen carries a negative charge, bonding to a less electronegative or more electropositive element. In most familiar compounds like water, hydrogen gives up its electron and sits at an oxidation state of +1. In a hydride, the opposite happens: hydrogen gains an electron to become H⁻, giving it an oxidation state of -1. This simple reversal of hydrogen’s usual role creates a family of compounds with surprisingly diverse properties and uses.
How Hydrogen Becomes Negative
Hydrogen normally loses its single electron when bonding with elements like oxygen or chlorine. But when paired with metals that give up electrons even more readily, hydrogen becomes the one accepting. Sodium hydride (NaH) is a classic example: sodium, a Group 1 metal, always carries a +1 charge in compounds, so hydrogen must sit at -1 for the charges to balance. The result is an ionic crystal where sodium exists as Na⁺ and hydrogen as the hydride ion, H⁻.
This makes the hydride ion chemically unusual. It is one of the simplest anions possible, just a hydrogen atom with an extra electron. That extra electron makes hydrides highly reactive, especially around water, because the H⁻ ion is eager to donate that electron to almost anything that will take it.
Three Main Types of Hydrides
Hydrides fall into three broad categories based on how hydrogen bonds to the other element: ionic, covalent, or metallic. Some compounds blur these lines, but the categories help explain their very different behaviors.
Ionic (Saline) Hydrides
These form between hydrogen and the most electropositive metals, primarily the alkali metals (lithium, sodium, potassium) and alkaline earth metals (calcium, barium). They’re made by heating the metal directly in hydrogen gas. The result is a white, high-melting-point solid with a crystal structure similar to table salt, which is why they’re sometimes called saline hydrides.
Ionic hydrides react violently with water. Sodium hydride dropped into water produces sodium hydroxide and hydrogen gas. This reaction is vigorous enough to be hazardous, which is why these compounds must be stored away from any moisture. Evidence for their truly ionic nature comes from several directions: molten ionic hydrides conduct electricity (meaning charged particles are moving freely), and their crystal structures match what you’d predict for a compound made of two oppositely charged ions.
Covalent (Molecular) Hydrides
When hydrogen bonds to nonmetals or metalloids, it shares electrons rather than fully gaining or losing them. These covalent hydrides include many everyday substances: water (H₂O), methane (CH₄), and ammonia (NH₃). They tend to be gases or liquids at room temperature, a stark contrast to the solid ionic hydrides.
Chemists further divide covalent hydrides into three subcategories based on their electron count. Electron-deficient hydrides, like diborane (B₂H₆), don’t have enough electrons for conventional bonds and compensate with unusual bridging structures. Electron-precise hydrides, like methane and silane (SiH₄), have exactly the right number of electrons for standard bonds. Electron-rich hydrides, like water and ammonia, carry extra electrons as lone pairs, which is what gives water its ability to act as a solvent and ammonia its characteristic basicity.
Metallic (Interstitial) Hydrides
Transition metals and rare earth elements form a third type, where hydrogen atoms squeeze into the gaps within the metal’s crystal lattice. These interstitial hydrides often look and feel like metals: they conduct electricity, have a metallic sheen, and are hard solids. The hydrogen atoms sit inside small cavities in the lattice, sometimes in triangular or pyramidal pockets formed by surrounding metal atoms. Many of these compounds don’t follow neat chemical formulas because the amount of hydrogen absorbed can vary continuously, producing non-whole-number ratios like TiH₁.₇.
Why Hydrides Matter in Chemistry Labs
Complex hydrides are workhorses in organic chemistry. Lithium aluminum hydride (LiAlH₄) is one of the most versatile, capable of converting acids, esters, and aldehydes into alcohols, and amides into amines. These reductions often proceed at room temperature or below, finishing quickly with few unwanted side reactions. Chemists can even fine-tune the process: adding the compound slowly to the hydride gives a full reduction, while adding the hydride slowly to the compound can selectively reduce one part of a molecule while leaving other reactive groups untouched. This kind of selective control is essential for building complex molecules in pharmaceutical and materials research.
Sodium borohydride (NaBH₄), a milder cousin, handles simpler reductions where the full power of lithium aluminum hydride isn’t needed or would be too aggressive.
Hydrides in Batteries
Nickel-metal hydride (NiMH) batteries, found in hybrid cars and rechargeable consumer electronics, rely on a metal alloy that absorbs and releases hydrogen during charging and discharging. The negative electrode stores hydrogen within the metal lattice as a hydride. During discharge, that stored hydrogen reacts to release electrons, generating current. NiMH batteries offered a significant step up from older nickel-cadmium technology in both capacity and environmental impact, and they remain widely used even as lithium-ion batteries dominate newer devices.
Hydrides for Hydrogen Storage
One of the most active areas of hydride research involves storing hydrogen fuel in solid form. Compressing hydrogen gas into tanks is expensive and requires heavy, high-pressure containers. Metal hydrides offer an alternative: absorb the hydrogen into a solid material and release it with heat when needed.
Magnesium hydride (MgH₂) is a leading candidate because magnesium is cheap, abundant, and can theoretically store 7.6% of its weight as hydrogen. In practice, researchers have achieved around 6.5 to 7% storage capacity through techniques like ball milling, which breaks the material into nanoparticles to speed up absorption and release. The U.S. Department of Energy has set a target of 6.5% by weight for light-duty vehicles, and magnesium-based hydrides are approaching that mark. The main remaining challenge is temperature: most magnesium hydride systems need to be heated to around 275 to 300°C to release their hydrogen quickly, which adds engineering complexity to vehicle fuel systems.
The Universe’s First Molecule Was a Hydride
Hydrides have a history that stretches back to the beginning of the universe itself. When temperatures in the early cosmos dropped below about 4,000 K after the Big Bang, ionized hydrogen combined with neutral helium to form helium hydride (HeH⁺), considered the first molecular bond ever created. The destruction of helium hydride opened one of the earliest chemical pathways to forming molecular hydrogen (H₂), the most abundant molecule in the universe.
For decades, helium hydride was predicted to exist in space but never observed there. In 2019, researchers using NASA’s SOFIA airborne observatory finally detected it in a planetary nebula called NGC 7027, confirming a prediction that had stood for decades. The detection required instruments sensitive enough to pick out the faint signal of helium hydride from nearby emissions of other molecules in the same region of the spectrum.