Final answer:
The relative atomic mass of an element is derived using carbon-12 as a reference standard, with its mass precisely defined as 12 amu. By comparing the average atomic mass of an element against this standard, scientists obtain a unified scale to express atomic masses in a consistent and relative manner, as displayed in the periodic table.
Step-by-step explanation:
The relative atomic mass of an element, such as element Y, is calculated based on carbon-12 as a reference standard, defined as having a mass of exactly 12 atomic mass units (amu). This reference is chosen because carbon-12 is a stable isotope and is abundantly available, making it practical for scientists to use as a standard measure. To find the relative atomic mass of element Y, one multiplies the average mass of one atom of element Y by 12 and then divides by the mass of one atom of carbon-12. This calculation ensures that the relative atomic mass is a comparison to the standard carbon-12, providing a dimensionless number (atomic mass units are used for convenience and consistency but are not explicitly stated in periodic tables).
The fact that the mass of carbon-12 is defined to be exactly 12 amu means that when performing this calculation for carbon-12, the figure '12' does cancel out, reaffirming that the relative atomic mass of carbon-12 is indeed 12. This process helps to standardize atomic masses of all elements, hence allowing scientists and researchers to use the periodic table as a reliable tool for understanding the properties of elements relative to each other.
All other atomic masses are measured against this standard, hence the relative atomic mass for any other element reflects how many times more (or less) massive an atom of that element is compared to an atom of carbon-12.