Beryllium (Be). Diagram showing the nuclear composition and electron configuration of an atom of beryllium-9 (atomic number: 4), the most common isotope of the element beryllium. The nucleus consists of 4 protons (red) and 5 neutrons (yellow). Four 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. Beryllium is an alkaline earth metal in group 2, period 2, and the s-block of the periodic table. At room temperature and pressure it forms a brittle metal that melts at 1287 degrees Celsius.