A Level Chemistry OCR Practice Exam

What characterizes a radical in chemistry?

1. A species with paired electrons

2. A stable molecule

3. A species with an unpaired electron

4. A compound that cannot participate in reactions

The correct answer is: A species with an unpaired electron

A radical in chemistry is specifically characterized as a species that has an unpaired electron. This unpaired electron makes radicals highly reactive and unstable, as they seek to pair with other electrons to achieve a more stable electronic configuration. Radicals can participate in various chemical reactions, often initiating chain reactions, such as those found in combustion or polymerization processes. In contrast, species with paired electrons are generally much more stable and less reactive. Stable molecules typically have a complete octet and balanced electron configurations, which would rule out the possibility of being classified as a radical. Additionally, a compound that cannot participate in reactions indicates extreme stability, typically not relevant to radicals, which thrive on reactivity due to their unpaired electrons. Thus, understanding radicals involves recognizing their defining feature: the presence of an unpaired electron.