What happens to a radioisotope when it undergoes nuclear decay?

The radioisotope changes into another element/atom through the process of nuclear decay

Explanation:

Nuclear decay is a process in which an unstable radioisotope changes into another element through the emission of characteristic radiation which are essentially fast moving energetic particles. This could be in the form of alpha, beta or gamma particles.

For example a Uranium, U-238 radioisotope gets converted to Thorium Th-234 nucleus through the release of alpha particles as shown below:

²³⁸U → ²³⁴Th + ⁴He

yes the answer is total mass in unchanged   : )