Isotope: Encyclopedia II - Isotope - Variation in properties between isotopes

Isotope - Variation in properties between isotopes

In a neutral atom, the number of electrons equals the number of protons. Thus, different isotopes of a given element also have the same number of electrons and the same electronic structure. Because the chemical behavior of an atom is largely determined by its electronic structure, isotopes exhibit nearly identical chemical behavior. The primary exception is that, due to their larger masses, heavier isotopes tend to react somewhat more slowly than lighter isotopes of the same element. (This phenomenon is termed the kinetic isotope effect).

This "mass effect" is most pronounced for protium (¹H) vis-à-vis deuterium (²H), because deuterium has twice the mass of protium. For heavier elements the relative mass difference between isotopes is much less, and the mass effect is usually negligible.

Similarly, two molecules which differ only in the isotopic nature of their atoms (isotopologues) will have nearly identical electronic structure, and therefore have similar physical and chemical properties. The vibrational modes of a molecule are determined by its shape and by the masses of its constituent atoms. Consequently, isotopologues will have different sets of vibrational modes. Since vibrational modes allow a molecule to absorb photons of corresponding energies, isotopologues have different optical properties in the infrared range.

Although isotopes exhibit nearly identical electronic and chemical behavior, their nuclear behavior varies dramatically. Atomic nuclei consist of protons and neutrons bound together by the strong nuclear force. Because protons are positively charged, they repel each other. Neutrons, which are electrically neutral, allow some separation between the positively charged protons, reducing the electrostatic repulsion and stabilizing the nucleus. For this reason neutrons are necessary for two or more protons to be bound into a nucleus. As the number of protons increases, additional neutrons are needed to form a stable nucleus; for example, although the neutron/proton ratio of ³He is 1/2, the neutron/proton ratio of ²³⁸U is greater than 3/2. If too many neutrons or too few neutrons are present, the nucleus becomes unstable and subject to nuclear decay.

Adapted from the Wikipedia article "Variation in properties between isotopes", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki