a) Radioactivity is the spontaneous process by which an unstable atomic nucleus transforms into a more stable configuration by emitting radiation. This process involves the decay of the nucleus, releasing energy and particles such as alpha particles, beta particles, or gamma rays. b) An atom becomes unstable primarily due to an imbalance within its nucleus. The main factors contributing to nuclear instability are: • Neutron-to-proton ratio: For lighter elements, a stable nucleus typically has a neutron-to-proton ratio close to 1:1. For heavier elements, this ratio increases, often around 1.5:1. Nuclei with too many neutrons or too few neutrons relative to protons lie outside the band of stability and are unstable. For example, a nucleus with an excess of neutrons might undergo beta-minus decay, converting a neutron into a proton and emitting an electron. Conversely, a nucleus with too few neutrons might undergo beta-plus decay or electron capture. • Total number of nucleons: Very heavy nuclei (those with a large number of protons and neutrons, typically atomic number $Z > 82$) are inherently unstable. The strong nuclear force, which holds the nucleus together, acts over short distances, while the electrostatic repulsion between protons acts over longer distances. In large nuclei, the repulsive forces become significant enough to overcome the attractive strong force, leading to instability and often alpha decay. • Nuclear binding energy: The stability of a nucleus is related to its binding energy per nucleon. Nuclei with lower binding energy per nucleon are less stable. Unstable nuclei will decay to achieve a configuration with higher binding energy per nucleon. A schematic illustration would typically show a plot of the number of neutrons (N) versus the number of protons (Z), with a central "band of stability" indicating stable nuclei. Nuclei above this band have too many neutrons, and those below have too many protons, both leading to instability. Another illustration might depict a large nucleus with many protons experiencing strong repulsive forces.