Tiny Frog has a nice, detailed article analyzing the evidence from radionuclides about the age of the earth. The article compares what would be still on earth given its scientific age and if it were 6,000 years old as suggested by Young-Earth Creationists. The decay of radioactive elements is measured in half-lives, the amount that it takes one-half of the atoms to change into another element.
I chose to list all the radionuclides which have half-lifes of 300+ years for one reason: a half-life of 300 years is exactly 20 half-lifes on a planet that is 6,000 years old. Notice that none of the radionuclides between Nb-92 and Cf-249 exist on earth except for the radionuclides we’ve talked about. There are 43 short-lived radionuclides in that group. If any of them had been present on earth 6,000 years ago “when the earth was created”, they would still exist. But, none of them exist.