18:49, Fri 11 Jan
As we gear up to celebrate 150 years of the Periodic Table, here's a quiz question.

How many elements have only one isotope?
19:00, Fri 11 Jan
4
19:03, Fri 11 Jan
3?
Keeping Right On since 1981
19:08, Fri 11 Jan
19 with single stable isotope

two others have only one isotope that contributes to the standard atomic weight, but that isotope is radioactive. 

There are five others that have only one stable isotope, but also have a second, radioactive, isotope contributing to the standard atomic weight.
19:22, Fri 11 Jan
Wikipedia says 22. You say 19.

Anyway curious why any at all. Or why 19/22.
19:26, Fri 11 Jan
Haircut1000
Wikipedia says 22. You say 19.

Anyway curious why any at all. Or why 19/22.

You are confusing with the 22 Mononuclidic Elements?

[en.m.wikipedia.org]
19:28, Fri 11 Jan
150