The physical basis of the chemical periodicity governing the periodic table is the electron shell closures at each noble gas (atomic numbers 2, 10, 18, 36, 54, 86, and 118): as any further electrons must enter a new shell with higher energy, closed-shell electron configurations are markedly more stable, leading to the relative inertness of the noble gases. Since protons and neutrons are also known to arrange themselves in closed nuclear shells, the same effect happens at nucleon shell closures, which happen at specific nucleon numbers often dubbed "magic numbers". The known magic numbers are 2, 8, 20, 28, 50, and 82 for protons and neutrons, and additionally 126 for neutrons. Nucleons with magic proton and neutron numbers, such as helium-4, oxygen-16, calcium-48, and lead-208, are termed "doubly magic" and are very stable against decay. This property of increased nuclear stability is very important for superheavy elements: without any stabilization, their half-lives would be expected by exponential extrapolation to be in the range of nanoseconds (10−9 s) when element 110 (darmstadtium) is reached, because of the ever-increasing repulsive electrostatic forces between the positively charged protons that overcome the limited-range strong nuclear force that holds the nucleus together. The next closed nucleon shells and hence magic numbers are thought to denote the centre of the long-sought island of stability, where the half-lives to alpha decay and spontaneous fission lengthen again.