Atomic weights, also known as atomic masses, are assigned to elements based on a weighted average of the masses of their isotopes. Isotopes are variants of an element that have the same number of protons but different numbers of neutrons in their nuclei.
The atomic weight of an element is expressed relative to the atomic mass unit (u) and is typically reported as a decimal number. The atomic mass unit is defined as one-twelfth the mass of a carbon-12 atom.
The process of assigning atomic weights involves several steps:
Isotope abundance determination: The first step is to determine the relative abundance of each isotope of an element. Isotope abundance refers to the proportion of each isotope in a naturally occurring sample of the element. This information is obtained through various techniques such as mass spectrometry and isotopic analysis.
Isotopic mass determination: The next step is to measure the mass of each isotope of the element. Mass spectrometry is commonly used to determine the precise masses of isotopes.
Calculation of weighted average: Once the relative abundance and mass of each isotope are known, a weighted average is calculated. This average takes into account both the abundance and the mass of each isotope. The formula for calculating the atomic weight is:
Atomic weight = (Isotope mass1 × Abundance1) + (Isotope mass2 × Abundance2) + ...
The values for each isotope mass are multiplied by their respective abundances, and the results are summed to obtain the atomic weight.
Reporting of atomic weights: Atomic weights are reported with appropriate precision and are usually rounded to a decimal place. The values are often provided on the periodic table of elements.
It's important to note that atomic weights can vary slightly for a given element due to natural variations in isotope abundances in different samples. However, for most elements, the atomic weights are relatively stable and are considered constant values for practical purposes.
The International Union of Pure and Applied Chemistry (IUPAC) periodically reviews and updates the atomic weights of elements based on new experimental data and advancements in analytical techniques.