Potassium (K). Diagram showing the nuclear composition and electron configuration of an atom of potassium-39 (atomic number: 19), the most common isotope of this element. The nucleus consists of 19 protons (red) and 21 neutrons (yellow). 19 electrons (whiten) bind to the nucleus, with a single, relatively unstable electron in the outer shell (ring). The stability of an element's outer electrons determines its chemical and physical properties. Potassium is an alkali metal in group 1, period 4, and the s-block of the periodic table. In elemental form, it is a soft, very reactive metal that is less dense than water, melting at 64 degrees Celsius.