What is a monomer?

• A. A large polymer
• B. A small molecule which can join to form a polymer
• C. A solvent
• D. A catalyst

Answer: (B)

A monomer is a small molecule that can join with other similar molecules to form a polymer. When many monomer units link together, they create a long chain called a polymer. The ability to join comes from reactive sites on the monomer that form covalent bonds during polymerization, allowing the chain to grow. This is why the description “a small molecule which can join to form a polymer” is the best fit. It’s not a large polymer (that’s the product), not a solvent (which dissolves substances), and not a catalyst (which speeds up reactions). For example, ethene is a monomer that polymerizes to form polyethylene.