Wikimedia
Wikipedia: Atomic Radius
This site from the encyclopedia Wikipedia provides a basic definition for what the atomic radius is, and an explanation of what covalent and metallic radii are.
Sophia Learning
Sophia: Periodic Trends: Atomic Radius: Lesson 1
This lesson will explain how the atomic radius changes over representative elements across rows and up and down columns on the periodic table. It is 1 of 3 in the series titled "Periodic Trends: Atomic Radius."
Sophia Learning
Sophia: Atomic Radius: Lesson 2
This lesson will define atomic radius. It is 2 of 3 in the series titled "Atomic Radius."
ClassFlow
Class Flow: Periodic Table Trend
[Free Registration/Login Required] This flipchart helps students learn about the periodic trend of Atomic Radius of Elements across a Period and down a Group.
Royal Society of Chemistry
Periodic Landscapes: Atomic Radii
This website provides a short but detailed definition of atomic radii. Also shows the atomic radius plotted against atomic number.