When an atom is attracted to another atom because it has an unequal number of electrons and protons, the atom is called an ion
In a reactivity table , the most reactive element is placed at the top and the least reactive element at the bottom. More reactive metals have a greater tendency to lose electrons and form positive ions.

Ions are charged species they may be positively charged (cation) or negatively charged (anion). Reactivity table in many books is also known by activity series which is series in which metals are arranged in decreasing order of their reactivity .Thus most reactive metal is placed at the top of the series and least reactive metal is placed at bottom of the series .

The series is prepared by performing displacement reactions between metals and their salt solution.
Hydrogen is taken as standard and the metals above it in the series are more reactive than hydrogen and metals below it are less reactive than hydrogen.
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