If an atom has the electron configuration 1s22s22p 1 , how many electrons must it gain to achieve an octet?

According  to  octet  rule  element  has  eight  electrons  in  its valence shell.This  give it  the  same  electronic  configuration  as  a  group  18  elements.Atom  with  electronic  configuration  1s2 2s2 2p1  require  five  electrons  to  achieve  octet. This  is  because the  valence  electrons  are  3 in 2s2  and 2p1  and  hence require  5  electrons  to  be  filled.

Because electrons have negligible mass, to account for the mass of the isotope, there must be 16 neutrons, 16 neutrally charged, massive, fundamental particles present in the sulfur nucleus. and thus finally, if we have the 32s2− ion, there are 18 electrons, 16 protons, and 16 neutrons.