Why is atomic size of `Ca^(2+)` smaller than that of `K^(+)` ?

Explanation

`K^(+) and Ca^(2+) ` are isoelectronic (each contain 18 electrons), `K^(+)` contain 18 protons in its nucleus annd `Ca^(2+)` contains 20 protons. Number of protons being more in `Ca^(2+)` ion, the electrons in the outermost shell of `Ca^(2+)` ion wil experience greater attractive force by the nucleus compared to `K^(+)`. consequently `Ca^(2+)` has smaller ionic size that `K^(+)` ion.