Boron (B). Diagram showing the nuclear composition and electron configuration of an atom of boron-11 (atomic number: 5), the most common isotope of the element boron. The nucleus consists of 5 protons (red) and 6 neutrons (yellow). Five 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. Boron is a metalloid in group 13, period 2, and the p-block of the periodic table. Although difficult to prepare, at room temperature and pressure it forms a solid that melts at 2076 degrees Celsius.