Reaction where electrons are exchanged.
2 Na(s) + 2 H2O(l) 2 NaOH(aq) + H2(g)
An alternate approach is to describe how one reagent effects another.
Reducing Agent, a substance that causes another substance to be reduced. Na(s) Na+(aq) + 1 e-
Oxidizing Agent, a substance that causes another substnace to be oxidized. 2 H+(g) + 2 e- H2(g)