Parent Atom Definition in Chemistry and Physics

Radioactive Decay
By Kurt Rosenkrantz [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons 

A parent atom refers to the atom that undergoes radioactive decay in a nuclear reaction.

Also Known As: parent isotope

Examples: When U-238 decays into Th-234, the parent atom is U-238.