 Okay, let's do this problem now straightforward problem, which noble gas is iso-electronic to these particular ion so again, all you got to do is well While you knew the electron configuration and all of that stuff, but all you got to do is Know that when you add one electron to chlorine it gains an electron of course and attains the argon configuration Because remember all of these ions are the things that are trying to get an octet. So They want to get the quote-unquote noble gas configuration and then chlorine's case the noble gas configuration is the argon electron configuration So that's what it's iso-electronic to for iso-electronic means iso is the same as or whatever okay so sodium what's What electron configuration to do the thing by losing that It's 3s electron configuration. So iso-electronic