The arrangement of metals in a vertical column in the order of decreasing reactivities is called reactivity series of metals.
The general tendency of metals to lose electrons differs from one metal to another and the arrangement of metals in the decreasing order of their reactivity is called the activity series of metals or the reactivity series.
Potassium is at the top because it is most reactive and gold is at the bottom because it is least reactive. Hydrogen is included in the series although it is a nonmetal because it can form positive ions.
SALIENT FEATURES OF THE REACTIVITY SERIES:
The electropositive character decreases as we move down the reactivity series. Potassium and sodium react with oxygen to form superoxide but in the limited supply of oxygen, they form their own oxides.
The reducing power of metals decreases as we move down the reactivity series. Hence potassium is the strongest reducing agent.
The tendency of metals to get oxidized decreases as we move down the activity series. Therefore potassium gets oxidized most readily.
Also, as we move down the reactivity series the ability of metals to reduce water into hydrogen reduces. Potassium and sodium being highly reactive react with cold water to liberate hydrogen gas which gets ignited by the heat produced in the reaction.
All the metals above hydrogen in the activity series have the ability to reduce hydrogen ions from dilute hydrochloric or dilute sulphuric acid to liberate hydrogen gas.
A metal which is placed higher in the reactivity series can replace a metal below it in order to form the salt solutions of the latter metal. The larger the difference between the positions of two metals the faster the displacement reaction.
The oxides of metals like potassium, sodium, calcium, magnesium, and aluminum cannot be reduced by the common reducing agents.