Atomic weights are calculated based on the relative abundance of different isotopes of an element and their respective masses. Isotopes are variants of an element that have the same number of protons but different numbers of neutrons. Since the atomic weight is an average value, it takes into account the mass of each isotope and its abundance in nature.
The calculation of atomic weights involves the following steps:
Determine the masses of each isotope: The masses of different isotopes of an element are measured using techniques like mass spectrometry.
Determine the abundance of each isotope: The abundance of isotopes refers to the proportion or percentage of each isotope present in nature. This information is obtained through various means, such as analyzing the isotopic composition of naturally occurring samples.
Multiply the mass of each isotope by its abundance: For each isotope, its mass is multiplied by its abundance. This step accounts for the contribution of each isotope to the overall atomic weight.
Sum up the results: The products obtained in the previous step are added together to calculate the weighted average mass of all the isotopes.
The resulting atomic weight is usually expressed as a decimal number because it represents an average value. Since the abundances of isotopes can vary, the atomic weight is not always a whole number. The decimal places reflect the contribution of different isotopes, with the more abundant isotopes having a greater influence on the overall value.