Oxygen (O). Diagram showing the nuclear composition and electron configuration of an atom of oxygen-16 (atomic number: 8), the most common isotope of the element oxygen. The nucleus consists of 8 protons (red) and 8 neutrons (yellow). Eight 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. Oxygen, vital for life on Earth, is a non-metal in group 16, period 2, and the p-block of the periodic table. As a gas (O2) it forms 20 percent of the atmosphere. It liquefies at minus 219 degrees Celsius.