Primary structure is the fundamental arrangement of atoms and covalent bonds in a molecule. It describes the sequence of amino acids that make up a protein and the nucleotides that make up DNA. However, if you are writing a scientific paper, it is important to avoid repetition of the same phrase. You can use synonyms such as "elementary structure," "initial makeup," or "fundamental schema" to describe the primary structure of a molecule. Other possible synonyms include "basic configuration," "preliminary organization," and "core framework." These alternatives can help add variety to your writing and make it more engaging for readers.