What characterizes radioactivity?

Radioactivity is characterized primarily as a characteristic of materials that emit excess energy. This phenomenon occurs when unstable atomic nuclei release energy in the form of particles or electromagnetic radiation in an attempt to reach a more stable state. Materials that undergo this process do so because they have an imbalance in their nuclear structure, which leads to the emission of radiation as they seek stability.

The other options do not accurately define radioactivity. While some radioactive emissions can produce light, radioactivity itself is not intrinsically defined by light emission. Additionally, the assertion that radioactivity is a type of radiation that is harmless is misleading, as many forms of radiation associated with radioactivity can be harmful depending on exposure levels. Lastly, describing radioactivity as a method of energy transfer does not capture its core nature, which revolves around the transformation of unstable nuclei rather than just the transfer of energy. Thus, understanding radioactivity as an emission of excess energy from unstable materials provides a comprehensive insight into the topic.