Isoelctronic (Iso = same and electronic = electrons) species are either atoms, ions or molecules which has same number of electrons.
Potassium has atomic number 19, means it contains 19 electrons in neutral state. In question K⁺ is given, it has one positive charge means it has lost one electrons and the number of electrons contained by it are 18. And atom with 18 electrons in periodic table is Argon (Ar). Means, argon is our first answer.
Now we will use periodic table to locate atoms nearer to Argon atom, if behind (smaller atomic number than Argon) then it must have negative charge (addition of electrons) or if after Argon (larger atomic number) then must contain positive charge )loss of electron) to make its electrons equivalent to Argon.
So, our first answer is Argon = Ar
Secondly, if chlorine gains one electron Cl⁻
Third, when sulfur gains two electrons S⁻²
Fourth, Phosphorous gaining 3 electrons P⁻³
Fifth, Calcium loosing 2 electrons Ca²⁺
Sixth, Scandium loosing 3 electrons Sc⁺³
Result: Ar, Cl⁻, S⁻², P⁻³,Ca⁺²,Sc⁺³ and K⁺ are isoelectronic.
