A Level Chemistry OCR Practice Exam

What is meant by a repeat unit in a chemical structure?

1. An arrangement of atoms that occurs only once in the structure

2. A specific arrangement of atoms that occurs repeatedly in the structure

3. A unique chemical formula of a compound

4. A type of ionic compound

The correct answer is: A specific arrangement of atoms that occurs repeatedly in the structure

A repeat unit in a chemical structure refers to a specific arrangement of atoms that occurs repeatedly within the larger molecular framework. This concept is particularly significant in polymers, where the repeat unit is the fundamental building block that is connected to other identical units to form long chains. The nature of the repeat unit determines many physical and chemical properties of the polymer, including its strength, flexibility, and reactivity. For instance, in polyethylene, the repeat unit is the -CH2-CH2- sequence, which continues throughout the polymer chain. Understanding repeat units is essential for analyzing the properties and behaviors of various materials, especially in the context of organic and polymer chemistry. The other options do not accurately capture the definition of a repeat unit. Describing it as an arrangement that occurs only once neglects the essence of repetition fundamental to this concept. A unique chemical formula pertains to the overall composition of a compound, not its structural repetition. Additionally, identifying it as a type of ionic compound is misleading, as repeat units are relevant to a variety of compound types, including covalent networks and organic compounds. Thus, the success of option B in defining a repeat unit underscores its critical role in understanding molecular structures.