What is the formula of the ion formed when potassium achieves noble-gas electron configuration

Potassium is an alkali metal which means that it only needs to get rid of one electron to achieve a noble gas configuration.  Since it only needs to get rid of one electron the ion will be a cation with a charge of +1 which means the symbol will be K⁺.

I hope this helps.  Let me know if anything is unclear.