 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 also 126 for neutrons . <unk> 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 electromagnetic 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 be at the centre of the long @-@ sought island of stability , where the half @-@ lives to alpha decay and spontaneous fission lengthen again . 
