Answer:
[tex]A_{r}[/tex] Na = 22.989 769 28
Explanation:
For any element can vary the number of neutrons can differ; the same atom differing only by its number of neutrons is called an isotope. The mass number of an element is determined by its proton and neutron number:
Its relative atomic mass or atomic weight of an element is an average of the atomic masses of the element's isotopes. Each isotope's contribution to relative atomic mass is determined by its proportion in the sample.
For instance the relative atomic weight for the element, [tex]A_{r}[/tex]...
[tex]A_{r}[/tex] Na = 22.989 769 28
...this is standard, and can readily be found in entries in the periodic table,but the isotopes atomic mass may be near 25
