"Periodic Table Atomic Radius." Science Notes and Projects. Word Press, 24 May 2013. Web. 23 Apr. 2015.
"Periodic Table Trends." Khan Academy. Khan Academy, 2015. Web. 23 Apr. 2015.
"Periodic Table Trends." Periodic Table Trends. N.p., 19 June 2013. Web. 23 Apr. 2015.
Sarquis, Jerry. "Chapter 5." Modern Chemisry. By Mikey Sarquis. Orlando: Holt, 2012. 141-62. Print.