The use of atomic mass in calculations for the sample mass of an element provides information about the occurrence of that element in nature. Here's what it tells us:
Isotopic distribution: Elements can have multiple isotopes, which are variants of the element with different numbers of neutrons in their nuclei. Isotopes have different masses but share the same atomic number (number of protons). The atomic mass listed on the periodic table is usually an average value that takes into account the relative abundance of each isotope in nature. By using the atomic mass in calculations, you are considering the weighted average mass of all the isotopes present in a sample of the element.
Natural abundance: The atomic mass takes into account the natural abundance of different isotopes. Natural abundance refers to the relative proportion of each isotope found in nature. Some isotopes are more abundant than others, and their contribution to the average atomic mass is greater. By using the atomic mass, you are accounting for the natural occurrence of different isotopes and their relative abundances in the sample.
Variation in mass numbers: Since different isotopes have different mass numbers, using the atomic mass allows for the consideration of this variation. If you were to use the mass number instead of the atomic mass in calculations, you would only account for a specific isotope and ignore the presence and contribution of other isotopes. This is not representative of the true composition of the element in nature.
In summary, the use of atomic mass in calculations for the sample mass of an element tells us about the average mass of all isotopes present in nature and the relative abundance of those isotopes. It provides a more accurate representation of the element's occurrence and composition in natural samples.