An element - a substance that cannot be decomposed into two or more simpler substances by ordinary or physical means.
The word ordinary excludes the processes of radioactive decay, whether natural or artificial, and high-energy nuclear reactions that do transform elements into one another.
There are five types of radioactive decay: alpha emission, beta emission, positron emission, electron capture, and gamma emission.
Each type of decay emits a specific particle which changes the type of product produced. The number of protons and neutrons found in the daughter nuclei (the nuclei produced from the decay) are determined by the type of decay or emission that the original element goes through.
For information on each type of decay, see Libretexts.org's Decay Pathways.
For further reading, refer to hyperphysics.phy-astr.gsu.edu - Radioactivity.