Answer the following question given this paragraph: 

Both fluorine and lithium are highly reactive elements because of their number of valence electrons. Fluorine will readily gain one electron and lithium will just as readily give up one electron to become more stable. In fact, lithium and fluorine will react together as shown in the Figure below . When the two elements react, lithium transfers its one “extra” electron to fluorine.


Q: Both fluorine and lithium are highly reactive elements because of their number of what?


A:
valence electrons