For an isoelectronic series of ions, the ion that is the smallest is always 1. the ion with the most electrons. 2. the ion with the fewest protons.

the ion with the most electrons

Explanation:

Ionic size generally depends on weather the ions are negative or positive. Averagely, atoms become smaller on moving from left to right in the periodic table. A movement from left to right in the periodic table leads to a gradual increase in the number of electrons and decrease in atomic size. Li+, Be2+ and B3+ are all isoelectronic but B3+ is the smallest ion.

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