Potassium is a metal. Remember that metals are electropositive in nature. This implies that they give out electrons when they undergo ionic bonding with nonmetals.
Chlorine is a nonmetal. So the bond between chlorine and potassium is ionic. Potassium gives out one electron to chlorine and the both ions now attain a stable octet.
The electronic configuration of potassium is [Ar]4s1. After giving out an electron to chlorine in an ionic bond, its electronic configuration is now [Ar].
