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

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.

If a hydrogen atom is neutral, it must also have  one electron. an oxygen atom (o) has an atomic number of eight (8). this means a neutral oxygen atom has eight protons and eight electrons. the element actium (ac) has an atomic number of 89, so it has 89 protons and 89 electrons in a neutral atom