Activity Series of Metals is used to predict whether a reaction is possible or not in single replacement reaction.  The metals including hydrogen are arranged from the most reactive metal down to least reactive metal.  A metal can only replace an element below it.

Li           Lithium          Most Active Metal
Rb          Rubidium        
K            Potassium
Cs          Cesium
Ba          Barium
Sr           Strontium
Ca          Calcium
Na          Sodium
Mg         Magnesium
Al           Aluminum
Mn         Manganese
Zn          Zinc
Cr           Chromium
Fe           Iron
Ni          Nickel
Sn          Tin
Pb          Lead
H  -        Hydrogen
Cu          Copper
Hg          Mecury
Ag          Silver
Pt            Platinum
Au          Gold          Least Active Metal


Based from the Activity Series of metals Lithium is the most active metal while Gold is the least active metal.  Metal can only displace or replace metal below it.  Example, metals above hydrogen are the metals that can displace hydrogen and the metals below hydrogen.  While metals below hydrogen are the metals that hydrogen can displace.