Final answer:
A synthesis reaction is the building up of a chemical compound by the union of its elements or from other suitable starting materials.
Step-by-step explanation:
A compound is a substance that consists of two or more elements chemically combined in a fixed proportion. The building up of a chemical compound, by the union of its elements or from other suitable starting materials, is called a synthesis reaction. In a synthesis reaction, elements or compounds undergo a reaction and combine to form a single new substance. This is represented by the chemical equation: 2Na(s) + Cl2(g) → 2NaCl(s).