Examples of such long-lived radioisotopes include -14, may at first seem puzzling.The explanation of the apparent paradox is that nuclides in this category are continually replenished by specialized nuclear processes: by the slow decay of uranium in the Earth in the case of radon and by the interactions of with the atmosphere in the case of carbon-14.The last term, the so-called pairing energy, takes on any one of three values depending on whether He) by emitting an electron.
Accordingly, it is important and useful to measure stability in more quantitative terms.
A uniform scale of nuclear stability, one that applies to stable and unstable isotopes alike, is based on a comparison of measured isotope masses with the masses of their constituent electrons, protons, and neutrons.
When used in date ranges, circa is applied before each approximate date, while dates without circa immediately preceding them are generally assumed to be known with certainty.
The great importance of the atomic number derives from the observation that all atoms with the same atomic number have nearly, if not precisely, identical chemical properties. Similarly, mesothorium was shown to be chemically indistinguishable from .
These substances were thought to be elements and accordingly received special names.
The lexicon of isotopes includes three other frequently used terms: had been found to contain small quantities of several radioactive substances never before observed.For example, the isotope C, which has a particularly stable nucleus, has an atomic mass defined to be exactly 12 amu.The total separate masses of 6 electrons and 6 protons (treated as 6 hydrogen atoms) and of 6 neutrons add up to 12.09894 amu. The numerical values of these terms do not come from theory but from a selection process that ensures the best possible agreement with experimental data.Circa (from Latin, meaning 'around, about'), usually abbreviated c., ca or ca. or cca.), means "approximately" in several European languages including English, usually in reference to a date.Circa is widely used in historical writing when the dates of events are not accurately known.The second term corrects the first by allowing for the expectation that nucleons at the surface of the nucleus, unlike those in the interior, do not experience forces of nuclear attraction equally from all sides.