Isotopes and average atomic mass

Most elements exist naturally as a mix of isotopes, atoms with the same number of protons but different numbers of neutrons, and therefore different masses. The atomic mass shown on the periodic table is a weighted average of those isotopes, based on how common each one is. For example, natural chlorine is about 76 percent chlorine-35 and about 24 percent chlorine-37, so its average atomic mass is about (0.76 x 35) + (0.24 x 37), or roughly 35.5 atomic mass units. That is why most atomic masses on the periodic table are not whole numbers, and why the average sits closer to the mass of the more common isotope.

This explanation is AI-generated and can occasionally be wrong. 🚩 Report