Chapter 2 - Atoms, Molecules, and Ions - Exercises - Page 76: 2.57c

Charge: +1

Atoms of elements want to have the electron configuration of the nearest noble gas (in terms of number of electrons) on the periodic table. Potassium (K) is in group 1A, so it has to lose one electron in order to have the electron configuration of the nearest noble gas, Argon. Thus, the charge of the most stable potassium ion will be +1.