back arrow   forward arrow

Key Idea 3
Percent Abundance of Isotopes and Atomic Weight
Concept Question: Do all samples of an element have the same isotopic abundance ratios?

What do we know?

 Water droplets on bamboo

To answer the question of the abundances of different isotopes, we will return to our discussion of heavy water and light water. Water molecules are composed of two hydrogen atoms and one oxygen atom. Hydrogen atoms have two stable isotopes, while oxygen atoms have three stable isotopes. However, the abundances of these isotopes in nature differ; they are not present in equal amounts.

Hydrogen’s most abundant stable isotope is hydrogen-1, with a natural abundance of 99.985%. Hydrogen-1 contains one proton and no neutrons. The less common stable isotope of hydrogen is hydrogen-2 (also called deuterium and sometimes abbreviated as D), which contains one proton and one neutron and has a natural abundance of only 0.015%. A third isotope of hydrogen, which is not stable, is hydrogen-3 or tritium (sometimes abbreviated as T). Hydrogen-3 is a radioisotope and is only found in trace amounts in nature.

Oxygen atoms are most often found in nature as oxygen-16 (eight protons and eight neutrons) with an abundance of 99.757%.  It can also be found as the less common oxygen-18 (eight protons and ten neutrons) that has a relative abundance of 0.205% or the very rare oxygen-17 (eight protons and nine neutrons) that has a relative abundance of only 0.038.

Interestingly, the isotopic abundances given above are not constant throughout the Earth. The isotopic abundances in molecules of a substance may vary for different samples as a result of a variety of physical or chemical transformations, such as those taking place in the water cycle on our planet.