Answer :
Noble gases are elements with complete octet configuration
this means their outershells are complete with 8 electrons
other elements have incompletely filled outershells therefore they either lose or gain electrons to gain a complete octet in the outershell
1. Na⁺ - Na electron configuration is 2,8,1
positive charge means it loses its outermost shell electron and becomes 2,8 with 8 electrons in the outermost shell
therefore has a noble gas configuration
2.Mg²⁺ - Mg electron configuration is 2,8,2
2 plus means Mg loses 2 electrons in the outermost shell and becomes 2,8 and has 8 electrons in the outer energy shell
therefore has a noble gas configuration
3.Ar - Ar electron configuration is 2,8,8
Ar has 8 electrons in the outermost energy shell therefore its complete
Ar element is a noble gas
4.S - S electron configuration is 2,8,6
S has 6 electrons in the outermost energy
therefore it does not have a noble gas configuration
correct answer is 4) S
