Fluorine (F). Diagram showing the nuclear composition and electron configuration of an atom of fluorine-19 (atomic number: 9), the most common isotope of the element fluorine. The nucleus consists of 9 protons (red) and 10 neutrons (yellow). Nine electrons (white) bind to the nucleus, successively occupying available electron shells (rings). The stability of an element's outer electrons determines its chemical and physical properties. Fluorine is a halogen in group 17, period 2, and the p-block of the periodic table. In elemental form it is a reactive, extremely toxic gas (F2). It condenses into a yellow liquid at minus 188 degrees Celsius.