# Why are sulfur trioxide and nitrate not isoelectronic, even though both have the same number of outer electrons?

Though $$\ce{SO3}$$ has the same number of outer electrons as $$\ce{NO3-}$$, the two are not isoelectronic.

This statement is from JD Lee, but I could not understand why is he calling these two molecules as not isoelectronic because ultimately being isoelectronic means having the same number of electrons in outermost shell, which they clearly do! Please explain.

• I thinks its just a difference of usage. Though not as common, it wouldn't be unreasonable to read "isoelectronic" as meaning "the same total number of electrons". Of course, it leads to confusion when it isn't properly explain. – Tyberius Aug 5 at 16:12
• You need to compare the electron configuration, not just the number of electrons! – Karl Aug 5 at 19:01
• Isoelectronic species are those which have similar electronic configuration. for this total no. of electrons should be same not only valence electrons. – Abhishek Ambast Sep 1 at 13:32