Combination Reactions
One of the simplest types of redox reactions is the combination reaction. In
these reactions, which involve the "combining" of two elements to form a chemical
compound, one element is always oxidized, while the other is always reduced as
illustrated below.
Example - Formation of sulfur trioxide from oxygen and sulfur.

Note: Hydrogen is oxidized and oxygen is reduced.
Example - Formation of sulfur trioxide from oxygen and sulfur.

Note: Sulfur is oxidized; oxygen is reduced.