Ionic bond

among proton, neutron, electron, only proton no.(atomic no.) distinguishes different elements.
for chlorine atom, upon reduction, gaining one electrons, electronic configuration changes from 2,8,7 to 2,8,8
however, no of protons remains 17 but does NOT increase to 18.
therefore, the product is not argon (atomic no=18)
as no of electrons of product=2+8+8=18
which is greater than no of proton of Cl by 1
therefore, the product is Cl-