Potassium has one valence electron. Valence electrons are the electrons found in an atom's outer energy level.
Potassium has an atomic number of 19, which means one neutral atom of this element has 19 electrons. There will be two electrons in the first energy level, eight in the second level, eight in the third level and one in the final energy level. This last electron is known as a valence electron.
It is possible to determine the number of valence electrons in an element by looking at the periodic table. The number of valence electrons corresponds to an element's group number. The vertical columns on the periodic table are also called groups. Because potassium is in group one, it can be assumed that it has one valence electron.