Some atoms, especially those of large mass, disintegrate spontaneously, manifesting radioactivity.
Pierre Curie and Marie Curie, the Curie couple, studied the radioactivity of uranium salts. They found that all uranium salts had the property of impressing photographic plates. They concluded that the responsible for the emissions was uranium (U).
They have done many experiments extracting and purifying uranium (U) from pechblenda ore (U3O). They observed that the impurities were more radioactive than uranium itself. In 1898 they separated from impurities a new chemical element, polonium (Po), named after Marie Curie's homeland, Poland. Polonium is 400 times more radioactive than uranium.
More experiments were done by the couple and another chemical element was discovered, Radio (Ra), 900 times more radioactive than uranium. This element turns bluish when it is in the dark and fluoresces some substances like ZnS, BaS, etc.
Atoms of radioactive elements are very unstable. For this reason, radioactivity is manifested by the emission of particles from the nucleus of the atom or electromagnetic radiation.
Nuclear Disintegration or Decay - Process where unstable nuclei emit particle and electromagnetic waves to achieve stability.
Only the element that has its unstable core is radioactive. The stability of the atomic nucleus is determined by the number of mass (A), ie amount of protons plus neutrons. Stability is only broken in atoms with very large mass numbers. From polonium (Powder-84), all elements have instability.
There are some lighter atoms with unstable nuclei in minimal proportions. They are called radioactive isotopes or radioisotopes.