Field of Science

Isotope Counts

In my quantum chemistry class we are exploring molecular vibrations. The characteristic frequency of vibration depends on the masses of the atoms in the vibrating bond. Not every student in my class is a chemist, a fact that was driven home when I assigned a problem asking students to compute the fundamental vibrational frequency of an molecule with a deuterium atom.

Most elements have several naturally occuring isotopes - forms of the element which have the same number of protons,but varying numbers of neutrons. For example, the most abundant form of the element carbon has a mass number (the sum of the number of protons and neutrons) of 12. One percent of carbon atoms, however, have an extra neutron and a mass number of 13. Carbon-14 has two extra neutrons, and is radioactive. Chemists often use isotopic substitution as a way to "tag" molecules (particularly if the isotope is radioactive).

Deuterium is an isotope of hydrogen - much of its chemical behavior is exactly the same as that of hydrogen,it just weighs more, twice as much to be precise. Unlike other elements, where the isotopes are designated by their usual name plus their mass number, isotopes of hydrogen get their own names. Deuterium has a mass number of 2, tritium a mass number of (surprise) 3. Tritium is radioactive and has a half-life of around 12 years.

In 1955, in his novel "The Mouse that Roared", Irish writer Leonard Wibberly coined "quadium" for hydrogen-4, which at the time had not been made. Since then, hydrogen-4 has been created. It is a fleeting species, its half-life is just over 10-22 seconds!


  1. Thanks for the reference to heavier isotopes.

    The overall chemistry of deuterium isn't actually very different. What is different is the speed at which the reactions proceed. That is why drinking a glass of heavy water is not such a great idea, it will slow down the rates of some reactions in your body.

    The reactions themselves don't change, however. If it was proton tranfer before, it'll still be heavy proton transer. This is why chemists can use isotopes as labels. The reaction proceeds by the same mechanism, but now you can follow the "bouncing ball" (or proton, or carbon or...) as it moves around between molecules. This can provide a lot of insight into the mechanism of the reaction.

  2. Mitch...I turned off the required registration, so subsequent commenters won't need to register.

  3. I guess it depends if you consider the term chemistry to be solely based on the overall delta G, or as both the delta G and kinetics(speed) combined.

    Now, this is just semantics, since we both know what we are talking about. :)


  4. I guess my stable side is showing! Someone should design a chemistry Myers-Briggs. Instead of Introvert/Extravert it could be Kinetics/Thermodynamics. ;-)

  5. Thanks for the link to, i had no idea that isotopes went that far up.

  6. sorry, i mis-spelled


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