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Predicting Ionic Charge from Group Position

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State

A main-group metal's cation charge and a main-group nonmetal's anion charge both follow from reaching the nearest noble-gas electron configuration.

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Magnesium, in group 2, loses two electrons to form Mg²⁺; oxygen, in group 16, gains two electrons to form O²⁻.

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This group-position rule predicts main-group ions reliably — many transition metals form more than one stable charge and resist it.