Online Introductory Chemistry

Isotope definition and average atomic masses

Dr. Walt Volland revised March 31, 2005
 
Isotopes: All the atoms of an element have the same atomic number, but they can have different numbers of neutrons and different mass numbers. Isotopes of an element are atoms thathave the same number of protons, but different numbers of neutrons. Examples of isotopes are the three different kinds of carbon atoms where all have 6 protons, but different numbers of neutrons specifically 8, 7 and 6 neutrons respectively.
          • Carbon-14; 8 neutrons
          • Carbon-13; 7 neutrons
          • Carbon-12; 6 neutrons
Greek "iso" means same and "topos" means place. This fits the idea that isotopes are in the same place in the periodic table, but have different masses. Periodic table entries provide the information shown here. The periodic table does not indicate isotope information.

 

Isotope abundances:

The isotopes of an element do not occur with equal frequency. The relative abundance depends on the relative stability of the isotope. The isotopes contribute to the average atomic mass based on their abundance. The result is that the "average" mass for the atoms of an element is dictated by the most abundant or common isotope. The atomic weights in the periodic table are weighted averages. This means the tabulated value doesn't match any actual atom, but is closer to the most common isotope. The weighted average is calculated in the same way you figure out your grade after a series of assignments.
An example calculation is on the next page.