Why is the Relative Atomic Mass of Oxygen Less than 16?

Oxygen has three naturally occurring isotopes (see table below). The relative atomic mass of oxygen is 15.9994. Why is the relative atomic mass of oxygen less than 16?

Isotope % Natural Abundance
oxygen-16 99.762%
oxygen-17 0.038%
oxygen-18 0.200%

The atomic mass of an element is defined as the average of the isotopic masses, weighted according to the naturally occurring abundances of the isotopes of the element. Although oxygen has nine isotopes with mass numbers ranging from 13 to 21, oxygen-13, oxygen-14 and oxygen-15 are not naturally occurring isotopes. So, we do not need to include them when we calculate the relative atomic mass of oxygen. Why is the relative atomic mass of oxygen less than 16?

Actually, we cannot determine the mass of an atom just by adding up the masses of its protons and neutrons. Thus, the following calculation is incorrect:

Relative atomic mass = 16 x 99.762% + 17 x 0.038% + 18 x 0.200% = 16.008

The actual masses of individual oxygen atoms are not whole numbers even though their mass numbers are whole numbers. When protons and neutrons combine to form the oxygen nucleus, a very small amount of the total mass is converted to nuclear binding energy. However, we cannot predict the amount of nuclear binding energy just by counting the numbers of protons and neutrons. It must be experimentally determined using a mass spectrometer. For example, since the mass of one atom of oxygen-16 is 1.33291 times the mass of one carbon-12 atom, the mass of one oxygen-16 atom should be equal to 1.33291 x 12 = 15.99492 atomic mass units.

It is important to note that only the mass of one carbon-12 atom is a whole number because, by international agreement, one atom of the carbon-12 isotope has been assigned a mass of 12 atomic mass units.

The concept of isotopic mass is not clearly described in many textbooks. Writers usually emphasize that the relative mass of an electron is close to zero and so the relative mass of an isotope is approximately equal to its mass number. This kind of explanation may result in student misconceptions of atomic mass.

David Dice has developed an interesting experiment with atomic mass (http://www.carlton.paschools.pa.sk.ca/chemical/Molemass/moles3a.htm). You may adapt it to meet the needs of your chemistry students.