Rubidium (Rb). Diagram of the nuclear composition and electron configuration of an atom of rubidium-85 (atomic number: 37), the most common isotope of this element. The nucleus consists of 37 protons (red) and 48 neutrons (yellow). 37 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. Rubidium is an alkali metal in group 1, period 5, and the s-block of the periodic table. In elemental form, it is a soft, extremely reactive metal that is denser than water, melting at 39 degrees Celsius.