It is a series in order of the standard electrode potentials.  the negative potentials are placed at the top while positives are placed below n hydrogen is placed in the middle . more negative , more it is the reducing agent . this series is called electrochemical series.
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   1) The electrochemical series is built up by arranging various  redox equilibria in order of their standard electrode potentials.                                                        2) The most negative  values are placed at the top and positive values are placed at the bottem of  the  electrochemical series.


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