Final answer:
An atom is radioactive due to instability in its nucleus, which can break apart over time due to insufficient nuclear binding energy, leading to radioactive decay and transmutation into another element.
Step-by-step explanation:
An atom becomes radioactive primarily due to instability in its nucleus. The correct answer to the question is 'C. Its nucleus can easily break apart over time.' A stable atomic nucleus has an optimal ratio of protons to neutrons and a sufficient amount of nuclear binding energy to hold the nucleus together against the electrostatic repulsion between protons. When the binding energy is insufficient due to an imbalance in the number of protons and neutrons, the nucleus may undergo radioactive decay as it seeks a more stable configuration. This can involve the emission of particles such as alpha (helium nuclei) and beta particles (electrons), as well as gamma rays, leading to the transformation of the atom into a different element, a process known as transmutation.
All elements with atomic numbers greater than 83 are radioactive, meaning they do not have any stable isotopes, and they undergo spontaneous decay over time. For elements with fewer protons, some isotopes are stable while others are not. The forces that keep the nucleus together are the strong nuclear force, which is much stronger than the electrostatic force repelling the protons, and the weak force, which is involved in certain types of nuclear decay processes.