Stable and Unstable Isotopes
That's not to say that an element can't have more than one stable isotope.
Ordinary hydrogen and deuterium are both stable; tin has ten stable
isotopes, more than any other element. Conversely, there are some elements
that don't have any stable isotopes at all. Elements 43 and 61, technetium
(Tc) and promethium (Pm), were missing from Mendeleev's original periodic table for this
reason. All the elements beyond bismuth (atomic number 83) are
unstable as well.