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Hydride refers to a chemical compound that contains at least one hydrogen atom and has hydrogen in the -1 oxidation state. In these compounds, hydrogen acts as a ligand or a component of an anionic group. Hydrides are broadly classified into three main categories: ionic hydrides, covalent hydrides, and metallic hydrides.

Ionic hydrides are typically formed between hydrogen and highly electropositive elements, primarily alkali and alkaline earth metals. These compounds are often crystalline solids and are non-volatile. They react vigorously with water to produce hydrogen gas and the corresponding metal hydroxide. For example, sodium hydride (NaH) is an ionic hydride.

Covalent hydrides are formed when hydrogen bonds covalently with other nonmetals, such as carbon, nitrogen, oxygen, or halogens. These compounds exhibit a wide range of physical properties, from gases like methane (CH4) and ammonia (NH3) to liquids and solids. Their polarity depends on the electronegativity difference between hydrogen and the other element. Water (H2O) and hydrogen chloride (HCl) are examples of covalent hydrides.

Metallic hydrides, also known as non-stoichiometric hydrides, are formed with transition metals and inner transition metals. In these compounds, hydrogen atoms occupy interstitial sites within the metal lattice. They are characterized by their metallic luster and electrical conductivity. The hydrogen content in metallic hydrides can vary, and they often do not follow simple stoichiometric ratios. Examples include palladium hydride (PdHx).

Hydrides play significant roles in various chemical processes, including as reducing agents, sources of hydrogen, and in energy storage applications. The study of hydrides is crucial for understanding chemical bonding and reactivity.