The Periodic Table and Isotopes (Abundance)

The periodic table only tells us a weighted average of the atomic masses of the different isotopes for an element. For example, if we take a weighted average for the isotopes of Carbon, we get an average atomic mass of 12.011 amu. Like Carbon, many elements exist in nature as a mixture of isotopes.

To find the average atomic mass of the element Carbon, we multiply the mass of each isotope by its percent abundance expressed as a decimal. The table below shows the exact mass of each isotope (isotopic mass) and the percent abundance (sometimes called fractional abundance) for the primary isotopes of Carbon.

Isotope Exact Weight
(Isotopic Mass)
Percentage Abundance
Carbon-12 12.0000 98.90 %
Carbon-13 13.0033 1.10 %

To find the average atomic mass for Carbon:

Average Atomic Mass = (12.0000)(.9890) + (13.0033)(0.0110) = 12.011 amu

The answer, 12.011 amu, is the same value found for Carbon on the periodic table.

Exercise: Calculate the average atomic mass of Nitrogen (N) based on the information given.

Mass Number Exact Weight
(Isotopic Mass)
Percentage Abundance
14 14.003074 99.63%
15 15.000108 0.37%

The exact mass of an isotope is something that is determined experimentally. To find the average atomic mass experimentally a mass spectrometer is used. This laboratory instrument works based on the differences in the mass of the different isotopes of an element. Because isotopes for an element have different masses, they can be separated, and the percentage of each isotope measured. From these measurements the average atomic mass can be calculated.

Review of Important Terms:

  • Mass Number is the number of protons and neutrons in an isotope. It is a whole number. We use mass number in naming isotopes, like Carbon-12 or Oxygen-17.
  • Atomic Mass is the mass of the entire atom of an isotope. This includes electrons as well as a slight change in mass due to binding energy.
  • Average atomic mass is the weighted average of the isotopes for an element. This is what is listed below the element symbol on the Periodic Table.

Exercise: Use the simulation to compare the average atomic mass and determine why Hydrogen (1.0079) is close to a whole number but Chlorine (35.453) is not.

In the PhET simulation, click on "Mixtures", and at the bottom of the page select "Nature's Mix". On the periodic table provided, select H or Cl.

Next Up: Notation & Naming