The stability or decay of an atom is determined by several factors, including the stability of its nucleus and the energy levels of its electron orbitals. While the tunneling probability of a particle can be non-zero, the stability of an atom is typically determined by other considerations.
Atoms are classified as stable when the forces binding their nucleus and electrons are such that the atom does not undergo spontaneous decay or disintegration. This stability is attributed to various factors:
Nucleus Stability: The stability of the nucleus depends on the balance between the attractive strong nuclear force and the repulsive electrostatic force between protons. In stable atoms, this balance is achieved through the specific arrangement of protons and neutrons, forming a stable nuclear configuration. However, in some cases, nuclei may be unstable and undergo radioactive decay due to an excess of either protons or neutrons, resulting in the emission of particles or radiation.
Energy Levels: Electrons in an atom occupy specific energy levels or orbitals. Stable atoms have filled or partially filled electron orbitals, following the principles of electron configuration. The lowest energy state of an atom is called the ground state, and stable atoms have completely filled or half-filled electron shells, which confer added stability. These filled electron shells result in a lower probability of electrons tunneling to other states.
Conservation Laws: Certain conservation laws play a role in the stability of atoms. For example, the conservation of energy and the conservation of lepton and baryon numbers impose constraints on the allowed decay processes. If a decay violates these conservation laws, it is highly suppressed or forbidden, contributing to the stability of the atom.
It's important to note that the tunneling probability you mentioned in the context of quantum mechanics typically applies to individual particles, such as electrons, moving through potential barriers. In the case of atoms, stability is a collective property that involves the overall behavior and interactions of the atomic constituents.
While no atom is truly immune to all forms of decay or interaction, stable atoms are characterized by extremely long lifetimes and have negligible decay rates under normal conditions. They maintain their structural integrity and do not undergo spontaneous changes that would lead to their disintegration.