The article provides a well-written and complete description of the chemical compound, as described in How to write a Great Article. It includes the full standard infobox table of chemical and physical data, and an appropriate series of headings to break up the article. It should have sufficient external literature references, preferably from the "hard" scientific literature rather than websites. It should either have a properly drawn (ChemDraw or equivalent) structure for covalent compounds, or preferably a photograph if it is a simple ionic compound. It should be at the stage where it could at least be considered for featured article status.
The article is "almost there" but it may be missing one of the following: a data table, balance of content, or an important section. Alternatively, the English may need a comprehensive rewrite to make it flow.
The article is substantial, but is still missing important content or contains much irrelevant material. The article should have some references to reliable sources, but may still have significant problems or require substantial cleanup
The article has a good amount of content, but it is still weak in certain areas, and may lack a table. For example it may cover the uses and physical properties extensively, but be weak on actual chemistry.
The article is either a very short article or a stub that will need a lot of work to bring it to A-Class level, although there may be a significant amount of material in the article.