Hydrogen in the combined state, mainly as water, hydrocarbons
and other organic compounds, constitutes about 11 % of the earth's
crust by weight*. Hydrogen gas is not very reactive; it reacts
spontaneously with very electropositive elements (some ot the
metals of Groups I and II) and with the very electronegative element
fluorine; with other elements, reactions usually require a catalyst—
heat or light—and even then may be incomplete.