Atomic Radii of Isotopes
Name: John Eric M.
In a discussion in Sciforum I have not been able to get a clear answer on the atomic radii
of isotopes. Have they been measured or is it an assumption that they are the same as the element with
the same atomic number? I have check the library and web, where the isotope tables show only the mass.
I am aware that the outer electron orbits are the same for element and isotope but, I assume that as
atoms and particles are fields, the addition of a neutral field (neutron) could increase the volume
without altering the radii of outer electrons as the electron orbit is determined by the magnetic
To a good approximation the atomic radii of isotopes are all the same. The major interaction holding
atoms together is electrostatic attraction of the oppositely charged nucleus and the various electrons.
Since the neutron has no charge, there is no electrostatic attraction between an electron and a
neutral particle, the atomic radius will not change. There are other much weaker interactions
that can have a small effect -- spin on the electron interacting with the spin on the nucleus
(if it has one), orbital angular momentum of the electron interacting with spin on the nucleus
(if it has one). Possible relativistic effects too. However, other "minor" interactions have a
larger effect than these -- specifically electron / electron interactions. So while one could
calculate (I suppose) the isotope effect on atomic radius, it would be very small. I don't recall
ever seeing the results of such a calculation because it could only be done on atoms containing
very few electrons to not be "swamped" by the electron / electron effects.
There must be some small effect.
I do know of the effect of isotopes on the dimensions of crystals of atoms. The atoms all attach
themselves in a regular manner, a crystal structure. This mainly would have to do with the size of
the atom, its chemical bonding etc, but has a tiny isotope effect, see Physical Review B, Vol 38, No. 8,9/15/1988, Effect of isotope concentration on the lattice parameters of germanium perfect crystals,
R.C. Buscheret et. al.
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Update: June 2012