A radioactive atom has a nucleus that is not entirely stable and will sometimes undergo nuclear fission, which is the process of an atom splitting into two smaller nuclei. From the element iron and upward, there is progressively more energy involved in holding nuclei together. This is why there are so many radioactive isotopes in the heavier elements, but there are exceptions such as Technetium.
Based on random chance, do to the uncertainty principle, such an atom will suddenly lose a piece of its nucleus. The nucleus will either eject what is known as an alpha particle, being two protons and two neutrons, or a beta particle, being a highly charge electron.
In the first case, the atom loses 4 atomic mass units and changes into a lighter element with the 2 proton loss, for instance uranium turns into Thorium. In the second case of a beta particle, the situation still involves the nucleus, but what has happened is a neutron has decayed into a proton, electron, and neutrino. The electron is ejected and the proton stays in the nucleus, keeping the atomic weight unchanged but increasing the atomic number by 1. For instance the Thorium caused by decay of Uranium will turn into Protactinium.
In both cases the atomic nucleus loses mass but only in the case of the alpha particle is mass loss significant. A tremendous amount of energy is released in these reactions due to a principle called packing fraction. Less energy is needed to hold a smaller nucleus together so energy is released when the atom loses mass.
The same principle works in reverse for all atoms smaller than iron, which is the principle of nuclear fusion when energy is released from two atoms combining together.
Because this atomic decay is based on random chance, the rate of decay is measured in half lives, which is the amount of time required for half of a given sample to undergo nuclear fission (not the time taken for half the mass to disappear).
Each element has a half-life, a period of time. Every time that half-life occurs, half the mass of the element is lost. Basically, that goes on and on until the mass is so small, scientists consider the element to be gone.
