Why does the phenomenon of radioactivity serve as one of the proofs of the complex structure of the atom?

The phenomenon of radioactivity is the phenomenon of the transformation of the nuclei of atoms of one chemical element into the nuclei of atoms of another chemical element.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.