Lithium (Li). Diagram showing the nuclear composition and electron configuration of an atom of lithium-7 (atomic number: 3), the most common isotope of the element lithium. The nucleus consists of 3 protons (red) and 4 neutrons (yellow). Three electrons (white) 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. Lithium is an alkali metal in group 1, period 2, and the s-block of the periodic table. In elemental form, it is a soft reactive metal that is less dense than water, melting at 180 degrees Celsius.