In thinking about a title for this chapter, the word periodicity came to mind. I was sure this had some kind of pop culture reference. After discussing this with my wife, we figured I was thinking of synchronicity, which is a reference to music by the band, The Police. Looking the word Periodicity up on the internet, I found that I was, in fact, a science geek and had not made a hip reference. Periodicity refers mainly to the Periodic Table, which is a focus of this chapter. No music, just science.