The periodic table provides the average atomic mass for each element, taking into account the relative abundance of each isotope and their respective atomic masses.
Isotopes are variants of an element with different numbers of neutrons in their nuclei. While the number of protons (atomic number) remains the same for a particular element, the number of neutrons (and thus the mass number) can vary among different isotopes.
In nature, elements exist as a mixture of isotopes, and the abundance of each isotope can vary. Some isotopes may be more abundant than others. When determining the atomic mass of an element for the periodic table, scientists consider the masses of all naturally occurring isotopes and their relative abundance.
To calculate the average atomic mass, the atomic mass of each isotope is multiplied by its natural abundance (as a decimal) and then summed. This accounts for the contributions of each isotope to the overall atomic mass of the element.
For example, carbon has two common isotopes, carbon-12 and carbon-13, with natural abundances of approximately 98.9% and 1.1% respectively. The periodic table provides the average atomic mass of carbon, which takes into account these isotopes and their abundances.
So, the specific mass of an atom of an element mentioned in the periodic table is an average value that considers the different isotopes and their relative abundances in nature. It represents the weighted average of the masses of all the isotopes of that element.