There are no known stable isotopes of the element with an atomic number of a 56 B 38 C100 D 20?

Element 20 is Tin.

Element 38 is Strontium.

Element 56 is is Barium.

Element 100 Fermium. Fermium is artificially created, and has no stable isotopes.

In general, there are no stable isotopes heavier than Bismuth (element 83).