chem hw help!?

6. Which of the following is NOT isoelectronic with sulfide, S2-? 

a) Ca2+ 

b) Ar 

c) Cl− 

d) O2-

az_lender2021-04-07T13:09:00Z

The O2- is isoelectronic with Neon, not with Argon.  Look at the periodic table, it will be obvious!