Krypton (Kr). Diagram of the nuclear composition and electron configuration of an atom of krypton-84 (atomic number: 36), the most common isotope of this element. The nucleus consists of 36 protons (red) and 48 neutrons (yelow). 36 electrons (white) bind to the nucleus, filling the outer (fourth) electron shell in what is a very stable configuration. The stability of an element's outer electrons determines its chemical and physical properties. Krypton is a noble gas in group 18, period 4, and the p-block of the periodic table. An inert gas found in trace amounts in the atmosphere, it is used in fluorescent lighting and lasers.