This is the weighted average of all of the isotopes of an element.

Atomic mass

Explanation:

Atomic mass (Also called Atomic Weight, although this denomination is incorrect, since the mass is property of the body and the weight depends on the gravity) Mass of an atom corresponding to a certain chemical element). The uma (u) is usually used as a unit of measure. Where u.m.a are acronyms that mean "unit of atomic mass". This unit is also usually called Dalton (Da) in honor of the English chemist John Dalton.

It is equivalent to one twelfth of the mass of the nucleus of the most abundant isotope of carbon, carbon-12. It corresponds roughly to the mass of a proton (or a hydrogen atom). It is abbreviated as "uma", although it can also be found by its English acronym "amu" (Atomic Mass Unit). However, the recommended symbol is simply "u".

The atomic masses of the chemical elements are usually calculated with the weighted average of the masses of the different isotopes of each element taking into account the relative abundance of each of them, which explains the non-correspondence between the atomic mass in umas, of an element, and the number of nucleons that harbors the nucleus of its most common isotope.

In contrast, the atomic mass of an isotope does coincide approximately with the mass of its nucleons. This difference is due to the fact that the elements are not formed by a single isotope but by a mixture with certain abundances for each of them. While when we measure the mass of a particular isotope, we do not take abundance into account. In any case, not even the atomic mass of the isotopes equals the sum of the masses of the nucleons. This is due to mass defect.

the answer for sodium is 0.186

the answer for rubidium is 0.251