Question

Of the most common ions of the third period elements mg al p and s which is the largest

Answer 1

Answer is: P (phosphorus).
Magnesium form cation Mg²⁺, Mg lost 2 electrons, ionic radius decreases.
Aluminium form cation Al³⁺, lost 3 electron, radius decreases.
Phosphorous can form the phosphide anion P³⁻, gain 3 electrons, ionic radius increases.
Sulfur can form anion S²⁻, gain 2 electrons, but have one proton more than phosphorus, so smaller radius.