Page 31 - Radiochemistry and nuclear chemistry

20 Radiochemistry and Nuclear Chemistry

2.5. Physicochemical differences for isotopes 1
Although the isotopic variations in the heaviest elements can be attributed to the
consequences of radioactive decay, the variations observed in lighter dements are
attributable to chemical behavior. The rates and equilibria of chemical reactions depend on
the masses of the atoms involved in the reactions as is explained in w and w As a
consequence, isotopes may be expected to have somewhat different quantitative
physicochemical values (the isotope effect). As examples of the isotope effect, we may note
that the freezing point of H20 is 0~ (273.15 K), while that for heavy water D20 is 3.82
degrees higher (276.97 K). The boiling point of D20 is 1.43 K higher than that of H20.
Similarly, while H 2 boils at 20.26 K, D 2 boils at 23.59 K. As a result of these differences
in the boiling points, the vapor pressures at a particular temperature for H 2 and D 2 are not
the same and distillation can be used to fractionate hydrogen isotopes. Other physical
properties such as density, heat of vaporization, viscosity, surface tension, etc., differ
similarly.
The optical emission spectra for isotopes are slightly different since electronic energy
levels are dependent on the atomic masses. The light emitted when an electron falls from
an outer orbit of main quantum number n 2 to an inner orbit of quantum number n 1 (< n2)
is given by

= Ro~ Z 2 mre d (1/n12-1/n22)/me (2.13)

where ~ is the wave number (1/h m-l), Ro, is the Rydberg constant (1.097 x 107 m-l),
m e is the electron (rest) mass, and rare d the reduced mass, according to
-1 -1 -1
rare d = m e + mnucl (2.14)
where mnucl is the nuclear mass. For the light hydrogen isotope, the H a line (i.e. the
transition energy between n I = 1 and n 2 = 2) occurs at 656.285 nm, while the
corresponding line for the deuterium isotope of hydrogen occurs at 656.106 nm. This
difference could be predicted from (2.13) and its observation experimentally in 1932
provided the first evidence for the existence of a heavy hydrogen isotope. This spectral
difference has practical importance as it can be used for a spectroscopic analysis of the
amount of heavy water in light water. Similar isotopic line shifts occur in the spectra of all
elements, but are rarely as large as the shift of almost 0.2 nm observed for hydrogen. For
the isotopes 235U and 23Su, the isotopic shift is 0.025 nm.

2.6. Isotope effects in chemical equilibrium

In a mixture of molecules AX and BX, with a common dement X, an exchange of the
atoms of the common element between the two molecules may occur. When the two

1w167 outline the scientific basis for isotope effects and isotope separation. This is not essential for the
Radiochemistry part of this book.