Caption

Sodium (Na). Diagram showing the nuclear composition and electron configuration of an atom of sodium-23 (atomic number: 11), the most common isotope of the element sodium. The nucleus consists of 11 protons (red) and 12 neutrons (yellow). 11 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. Sodium is an alkali metal in group 1, period 3, and the s-block of the periodic table. In elemental form, it is a soft reactive metal that is slightly less dense than water, melting at 98 degrees Celsius.