A quantitative explanation of the isotope effect is not simple, exception H atom. For the heavier elements the effect is traced back to the change of nucleus radius with mass.
 is double that of .
Usual increase is not from .

In many cases the isotope effect is not sufficient to explain the hyperfine structure.
The number of hyperfine structure components is often considerably greater than the number of isotopes.
In particular, elements which have only one isotope in appreciable amount also show hyperfine structure splitting.
Likewise, the number of components of different lines is frequently quite different for one and the same element.
These hyperfine structures can be quantitatively explained, when it is assumed that the “atomic nucleus possess an intrinsic angular momentum with which is associated a magnetic moment”.
This angular momentum can have different magnitudes for different nuclei and also of course, for different isotope of the same element.
This is known as Nuclear spin