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!