A chemical property is one in which the chemical composition of a material changes. Generally, a chemical reaction is also involved in the process. An example of a chemical property is oxidation of a substance.
A physical property, on the other hand, does not need any change in composition and is observable. Examples of physical property include density or mass of a substance.
Radioactivity is the spontaneous generation of particles from an atom (or more specifically, its nucleus). The particles that are released, include alpha, beta and gamma particles, all of which causes changes in the number of sub-atomic particles in the atom. Generally, newer elements are formed as a result of radioactive decay. For example, radioactivity of uranium-238 generates thorium (an element of lower atomic number and mass than uranium) and an alpha particle. Since, changes at chemical level take place and since the composition of the material changes, radioactivity is a chemical property.
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