The species Ar, K and Ca contain the same number of electrons. In which order do their radii increase? (AIPMT 2015) (a) Ca Ar K (b) Ca K Ar (c) K Ar Ca (d) Ar K Ca

The trend of atomic radius increases down the group and decreases from left to right in the periods, In isoelectronic species, the higher the nuclear charge smaller will be the atomic radius. Argon (Ar), Potassium ion K , and Calcium ion 2 Ca have 18 electrons in each. Therefore, the correct order of ionic radii is 2 Ca < K Ar .