What describes the spontaneous breakdown of an atom?

The spontaneous breakdown of an atom is best described by radioactivity, which is the process in which unstable atomic nuclei decay over time, releasing energy and particles. This can happen in several forms, such as alpha decay, beta decay, and gamma decay, where the nucleus transforms into a different state or element. During radioactivity, the emission of radiation occurs without the need for external forces or reactions, making it a hallmark of unstable isotopes.

In contrast, nuclear fission involves the splitting of an atomic nucleus into smaller parts, typically triggered by an external neutron, which is not a spontaneous process. Thermal expansion relates to the physical increase in volume of materials when heated but does not involve atomic breakdown. Ionization refers to the process of removing an electron from an atom or molecule, turning it into an ion; while it involves changes at the atomic level, it does not describe the spontaneous decay of an atom's nucleus.

Therefore, the process of radioactivity accurately captures the concept of spontaneous nuclear breakdown.