Before generation
Use after ion identities and charges have been established, when learners need to coordinate charge neutrality, lowest whole-number subscripts, symbol case, and Roman-numeral meaning. It covers two monatomic ions only. It does not handle polyatomic ions, acids, hydrates, molecular prefixes, variable anion charge, stock-name parsing, historical names, parentheses, coordination compounds, or charge inference.
Worked example
Sodium one plus and chloride one minus form NaCl with no Roman numeral. Iron three plus and oxide two minus need two iron and three oxygen ions, Fe2O3, with III. Copper two plus and sulfide two minus reduce to CuS with II. Iron two plus and chloride one minus form FeCl2 with II, so the entered III is flagged.
Formula or methodology
For cation charge magnitude p and anion magnitude q, let g be their greatest common divisor. The neutral lowest-ratio formula uses q divided by g cations and p divided by g anions, omitting every subscript one. For a transition-metal row, the Roman numeral states the cation charge magnitude; it does not state the subscript or ion count.
How to interpret the result
Supported means exact element-symbol case, reduced subscripts, and the required Roman numeral all match the supplied charge evidence. A correct formula with a wrong numeral separates composition from naming evidence. Confirm that the chosen transition flag and charge are chemically valid before using the classification as feedback; the engine checks consistency, not periodic-table knowledge.
Common mistakes
Do not write charges as formula subscripts without reducing, carry plus or minus signs into a neutral formula, change element-symbol capitalization, write subscript one, use the anion charge as the Roman numeral, treat every metal as variable-charge, or infer correctness from pronunciation. Check total positive and negative charge and name cation before anion.