Why is hydrogen negatively charged in sodium hydride?

In sodium hydride (NaH), hydrogen carries a negative charge because it exists as a hydride ion ($H^{-}$). This occurs due to the significant difference in electronegativity between the electropositive sodium atom and the hydrogen atom.12

In this ionic bond, the sodium atom effectively transfers its valence electron to the hydrogen atom. Because sodium is much less electronegative than hydrogen, it loses its electron to form a positive sodium ion ($N{a}^{+}$), while the hydrogen atom gains that electron to complete its shell, resulting in the negatively charged hydride anion ($H^{-}$). This classification of sodium hydride as a saline or ionic hydride reflects this electron transfer, distinguishing it from covalent hydrides where electrons are shared between atoms of comparable electronegativity.2314