Cations are positively charged ions with fewer electrons than protons. To partake in reaction, metals lose electrons to achieve a stable octet configuration. It has lost valence electrons and wold have a positive charge associated with it.
Potassium is a group 1 element. A metal.
Potassium has 1 valence electron so it loses the valence electron to form a stable octet.
A potassium ion has a positive charge and therefore cannot be an anion but is a cation.
The group number pretty much denotes the number of valence electrons.
