This document discusses the periodic trends in covalent and ionic radii. It defines covalent radius as half the distance between two covalently bonded atoms. Covalent radii generally decrease down a group and increase across a period. Ionic radius is defined as the distance between the centers of neighboring cations and anions in an ionic bond. Ionic radii decrease down a group for cations and increase down a group for anions. Ionic radii also generally decrease with higher charges on the cation. Coordination number can also affect ionic radii, with larger numbers associated with larger radii.