Predicting the # of valence electron

Valence  electron:  electron which can take part chemical reactions also called the " reactable electron" in the outermost(energy level) open electron shell of atom.
-open shell: shell contains less than max. # of electron
-closed shell: contains exactly max. # of electron

valence electron: not in the core, not it d- & f- subshells

ex.
Al=[Ne]3s2 3p1  has 3 valence electron
Pb=[Xe]6s2  4f14  5d10  6p2  has 4 valence electron
Xe=[kr]5s2  4d10  5p6  or[Xe] has 0 valence electron (noble gases)

The periodic law summmarizes the period table
-The periodic law: properties of the chemical elements recur periodically when the elements are arranged from lowest to highest number.