Thoron was later identified as which radon isotope?

Thoron was later identified as radon-220, the radioactive radon isotope written as ²²⁰Rn.

While working at McGill University, Ernest Rutherford and Frederick Soddy investigated the radioactive emanation produced by thorium. They initially called this short-lived gaseous substance thoron. Its chemical behavior suggested that it belonged to the inert, or noble-gas, family rather than being an ordinary compound of thorium.

Later nuclear identification showed that thoron is radon-220. The name survives mainly as a historical term, while modern scientific usage generally specifies the isotope by its mass number. Radon-220 has a very short half-life—about 56 seconds—so it is produced and disappears quickly in the thorium decay series.

It is easy to confuse thoron with radon-222, the best-known radon isotope associated with uranium decay and indoor air concerns. Radon-226 is not an isotope of radon itself; it is a radium isotope that decays into radon-222. Thus radon-220 is the canonical answer to the question.