Chemical Properties

Reaction with oxygen. Hydrogen combines with oxygen to give water, in a strongly exothermic, explosive reaction -- the same reaction that is harnessed inside a hydrogen fuel cell to generate electricity:

Reaction with the halogens. Hydrogen also combines with all four halogens to give the corresponding hydrogen halide, but the ease and mechanism of the reaction vary sharply down the group: fluorine reacts explosively even in the dark; chlorine reacts at room temperature but needs light; bromine needs heating; and iodine's reaction is photochemical (light-driven).

In every one of these halide products, hydrogen sits in the +1 oxidation state.

Reaction with reactive metals. With strongly electropositive metals such as lithium, sodium and calcium, hydrogen instead ends up in the -1 oxidation state, forming ionic hydrides:

These simple metal hydrides are themselves useful as reducing agents in synthetic organic chemistry, and are also the starting materials for making other important complex hydrides, such as lithium aluminium hydride and sodium borohydride:

$4\,NaH + B(OCH_3)_3 \rightarrow Na[BH_4] + 3\,CH_3ONa$ …