potassium is an element found in group 1 in the periodic table all group 1 elements are metals and it has 1 valence electron in the outer most shell . A potassium atom is thus much more likely to lose the "extra" electron than to gain one
Potassium is not an alkaline earth metal, it is an alkali metal. The metals in the leftmost column of the periodic table. They are noted for their high reactivity and for the fact that most of their compounds are basic.