How are hydrides classified as ionic, metallic, covalent, or polymeric?

Hydrides are classified into four primary categories based on the nature of their chemical bonding and structure:

• Saline (Ionic) Hydrides: These compounds contain hydrogen as a negatively charged ion ($H^{-}$). They are typically formed by alkali metals and alkaline-earth metals at elevated temperatures. They are generally white crystalline solids that react vigorously with water to produce hydrogen gas.1
• Metallic Hydrides: Often called interstitial or alloylike hydrides, these are formed by transition metals and inner transition metals. They possess metallic characteristics such as luster and electrical conductivity, consisting of protons and metal atoms within an electron sea. They are often nonstoichiometric and are considered intermediate between salts and alloys.2
• Covalent Hydrides: These are primarily compounds of hydrogen and nonmetals, where atoms share electron pairs. They are typically volatile liquids or gases with low melting and boiling points, though properties can be modified by hydrogen bonding.3
• Polymeric Hydrides: Identified based on their structure, these hydrides (such as beryllium hydride) often feature bridging hydrogen atoms.41