It is believed that when an atomic structure attains the complexity of radium or uranium, for example, the internal electromagnetic forces are not sufficiently powerful in proportion to the mass of the atoms to hold the nuclei together. Thus, there is a slow but spontaneous disintegration of the atoms to more stable arrangements. As the nucleus disintegrates, there is a spontaneous release of energy. The curie is only a measure of an amount of radioactivity, it is not a measure of mass or weight. Bq You can expect to see the activity on your stock vials of radioactive material expressed in either type of unit - curie or becquerel.
Radioactive decay is a random process.
It cannot be affected or altered by any known physical or chemical processes. Decay is random in that it cannot be predicted which particular atoms are going to decay or when they will decay. However, the total number of atoms which will decay in a specified period of time is predictable. For each radioisotope, the half-life is constant. The units of half-life can vary from just a few seconds to tens of thousands of years.