STD 11 - 3. Classification of elements and periodicity in properties — JEE STD 11 Science — Question

$Cl ^{-}-1 s ^2 2 s ^2 2 p ^6 3 s ^2 3 p ^6$

It has 18 electrons and $17$ protons

Ar $-1 s^2 2 s^2 2 p^6 3 s^2 3 p^6$

It has 18 electrons and $18$ protons

$K ^{+}-1 s ^2 2 s ^2 2 p ^6 3 s ^2 3 p ^6$

It has 18 electrons and 19 protons

$Ca ^{2+}-1 s ^2 2 s ^2 2 p ^6 3 s ^2 3 p^6$

It has 18 electrons and $20$ protons

From this information, we can easily conclude that $Cl ^{-}$would be the largest because it has a higher number of electrons as compared to protons and so the 'effective nuclear charge' would be lower on each electron.