Mono-isotopic elements, by definition, consist of only one stable isotope. An isotope is considered stable if it does not undergo radioactive decay over a significant timescale. While mono-isotopic elements theoretically have an atomic mass that corresponds to the mass of their sole stable isotope, the fractional atomic masses often seen on the periodic table for these elements arise due to a more accurate consideration of isotopic abundances.
In reality, even mono-isotopic elements may have small traces of other isotopes, which may be radioactive or have very long half-lives. These minor isotopes exist naturally as a result of various processes, such as nuclear reactions in stars or through the decay of heavier elements. The presence of these isotopes, although in minuscule amounts, can be detected and measured with advanced analytical techniques.
To provide a more precise representation of the average mass of the atoms found in a natural sample of an element, the atomic mass of mono-isotopic elements is often given as a fractional value. This fractional atomic mass accounts for the contribution of the minor isotopes present in trace amounts.
For example, the atomic mass of hydrogen is listed as approximately 1.008 amu, even though it consists of a single stable isotope, hydrogen-1 (or protium). The fractional value accounts for the minute abundance of the heavier isotope deuterium (hydrogen-2), which is present at around 0.015% in naturally occurring hydrogen.
In summary, fractional atomic masses for mono-isotopic elements reflect the consideration of minor isotopes that exist naturally, albeit in tiny quantities. These fractional values provide a more accurate representation of the average atomic mass observed in natural samples of the element.