To find the atomic mass of an element, you can follow these steps:
Identify the isotopes: Determine the isotopes of the element and their respective abundances. Isotopes are atoms of the same element with different numbers of neutrons, resulting in different atomic masses.
Retrieve atomic masses: Look up the atomic masses of each isotope from the periodic table or reliable sources. The atomic mass is usually given in atomic mass units (amu).
Calculate the weighted average: Multiply the atomic mass of each isotope by its abundance (as a decimal), and then sum the results.
Atomic Mass = (Atomic Mass of Isotope A × Abundance of Isotope A) + (Atomic Mass of Isotope B × Abundance of Isotope B) + ...
For example, let's find the atomic mass of chlorine using the two isotopes chlorine-35 and chlorine-37. The abundance of chlorine-35 is approximately 75.77%, and the abundance of chlorine-37 is approximately 24.23%.
Atomic Mass of Chlorine = (35 amu × 0.7577) + (37 amu × 0.2423)
Performing the calculation:
(35 × 0.7577) + (37 × 0.2423) = 26.5195 + 8.9729
Atomic Mass of Chlorine ≈ 35.4924 amu
Therefore, the atomic mass of chlorine is approximately 35.4924 amu.
It's important to note that the atomic mass listed on the periodic table is often an average atomic mass considering the natural abundances of isotopes found on Earth. It may be slightly different from the calculated value due to variations in isotopic abundances in different samples or regions.