An element’s identity is defined by the number of protons in its atoms, known as the atomic number (Z). Atomic stability refers to the atom’s ability to exist indefinitely without undergoing spontaneous transformation. This longevity is determined by the internal structure of the atom’s nucleus, which dictates whether the element will persist over cosmic time scales.
Understanding Nuclear Stability vs. Chemical Stability
The term “stability” often causes confusion because it is used differently in chemistry and nuclear physics. Chemical stability describes an atom’s reluctance to participate in chemical reactions, which depends on its electron configuration. Noble gases are considered chemically stable because their outermost electron shells are full.
Nuclear stability refers to the resistance of the nucleus to spontaneous radioactive decay. A chemically reactive element, such as oxygen, is entirely nuclearly stable because its nucleus does not decay. Conversely, an element could be chemically inert but have an unstable, decaying nucleus. Therefore, when discussing stable atoms, the inquiry is fundamentally about nuclear stability.
The Inventory of Stable Elements
An element is defined as stable if it possesses at least one isotope whose nucleus does not undergo radioactive decay. The vast majority of naturally occurring elements, 80 out of the first 82, fall into this category, ranging from Hydrogen (Z=1) up through Lead (Z=82).
The list of stable elements has two notable gaps: Technetium (Z=43) and Promethium (Z=61). These elements have no stable isotopes and are entirely radioactive. They exist only fleetingly in nature or are synthesized in laboratories.
The element Bismuth (Z=83) represents the boundary of stability on the periodic table. Bismuth was historically considered the heaviest stable element, but in 2003, researchers discovered that its lone primordial isotope, Bismuth-209, is actually radioactive. Its half-life is an astounding \(2.01 \times 10^{19}\) years, meaning it is still treated as stable for almost all practical purposes. All elements with an atomic number greater than 83 are fundamentally unstable and exist only as radioactive forms.
The Principles Governing Atomic Stability
The stability of an atomic nucleus is a constant tug-of-war between two fundamental forces acting on the protons and neutrons (collectively called nucleons). The strong nuclear force is a powerful, short-range attractive force that binds all nucleons together. This attraction must overcome the electromagnetic force, which causes positively charged protons to repel every other proton in the nucleus.
The most important factor determining nuclear stability is the neutron-to-proton (N/P) ratio. For light elements, stable nuclei generally maintain an N/P ratio close to 1:1. As the atomic number increases, the growing electromagnetic repulsion requires an increasing number of neutrons to act as “nuclear glue.” This necessary ratio gradually climbs until it reaches approximately 1.5:1 for the heaviest stable elements, such as Lead.
When the nucleus is plotted on a chart of nuclides, the stable isotopes form a narrow region known as the “Belt of Stability.” Any isotope that falls outside this belt has an unbalanced N/P ratio and will undergo radioactive decay to achieve a more stable configuration.
Magic Numbers
Specific numbers of protons or neutrons, called “magic numbers,” confer extra stability to a nucleus, analogous to the stability of a filled electron shell. These numbers are 2, 8, 20, 28, 50, 82, and 126. Nuclei that possess a magic number of either protons or neutrons are more tightly bound and therefore more stable than their neighbors. If a nucleus has a magic number for both protons and neutrons, such as Lead-208 (82 protons and 126 neutrons), it is considered “doubly magic” and exhibits exceptional stability.