The basic unit of syntax is not the word, but the syntactic atom, defined as a structure that fulfills a basic syntactic function. Syntactic atoms may be either a single word or a phrase that fulfills a single syntactic function.
Fido ate the bone.
The dog ate the bone.
The big yellow dog ate the bone.
Our dog that we raised from a puppy ate the bone.
Elements with syntactic equivalence all belong to the same type of syntactic atom (NP, VP)