Final answer:
An unstable nucleus may lose energy and achieve stability by undergoing radioactive decay, where it emits radiation consisting of particles and energy, leading to the transmutation into a different element.
Step-by-step explanation:
At some point in time, an unstable nucleus will undergo a reaction and lose energy by emitting radiation, such as particles and energy. This nuclear process, known as radioactive decay, occurs because the nucleus seeks to become more stable. One example is the decay of uranium-235, which emits a helium nucleus (an alpha particle) to become thorium-231. In another form of decay, beta emission, a neutron is converted into a proton and an electron, with the electron being ejected as a beta particle. During radioactive decay, the total number of nucleons and the total positive charge are conserved, but the nucleus itself changes in composition, leading to the transmutation into a different element.