A constant (a superman in your example) and a variable are terms and terms alone are not atomic sentences. An atomic sentence can have a form like "Predicate(term)" [superhero(superman) in your example] or a term = term. Atomic sentences alone can be WFFs in first order logic (see below syntax).
I assume you can read a BNF form. Belows are the syntax of first order logic with equality. A "Sentence" below denotes a WFF in FOL.
//your example in Q