Final answer:
The calculated relative atomic mass might differ from the periodic table's value because the table averages the mass based on isotopes' percent natural abundance and their respective masses. This calculation also takes into account isotopic mixtures in nature, as opposed to any single isotope's mass number. For radioactive elements without stable isotopes, only the mass of the most common or stable isotope is listed.
Step-by-step explanation:
The calculated value of a relative atomic mass might differ from the value given in the periodic table due to the weighted average of the isotopic masses. When you calculate relative atomic mass, you must consider the percent natural abundance of each isotope. For instance, chlorine has two common isotopes, Cl-35 and Cl-37, with a higher natural abundance of Cl-35, making the average atomic mass lean closer to 35.
Furthermore, the periodic table reflects the weighted average atomic masses of isotopes as they naturally occur, not just a simple arithmetic mean. For example, carbon has an average atomic mass that takes into account the significant natural abundance of Carbon-12 and the lesser abundance of Carbon-13. This reflects why the periodic table lists an average atomic mass that may not match the mass number of any single isotope.
Lastly, some elements like technetium and promethium only exist as radioactive isotopes, so the atomic mass in the periodic table is given for the most stable or most common isotope, considering that no stable isotopes exist to take a proper average from.