Which pair of metals produces the highest voltage and why

The maximum voltage can be produced in the cell which have higher difference in the electrocemical potential , in this case it would be most reactive magnesium and less reactive platinum...magnesum spontaneously gives its two electron and Pt takes its two electron.... as result generates the highest voltage

periodic trends is an atom that gets larger as the number of electronic shells increase, the radius of atoms increases as you go down a certain group in the periodic table of elements. in general, the size of an atom will decrease as you move from left to right of a certain period.

yes

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