Chemical Formulas & Nomenclature

Why Formulas and Names Matter

A chemical formula shows the kinds and numbers of atoms in a substance; nomenclature is the system for naming it. These are among the most reliably scored skills on the NY Regents Chemistry exam because they follow fixed rules and use the Selected Polyatomic Ions table (the section the legacy edition labeled Table E, titled Selected Ions Forming Aqueous Solutions on the 2025 booklet). Master the rules once and you can write or name almost any compound, in both multiple-choice and constructed-response items.

A subscript is the small number that tells how many of the preceding atom or ion are present (the 2 in H2O means two hydrogen atoms). A coefficient, written in front of a formula, multiplies the whole formula and is used in equations, not in naming a single compound. Confusing the two changes the substance, so read subscripts carefully.

Building Ionic Formulas

Ionic compounds (metal + nonmetal) must be electrically neutral: total positive charge equals total negative charge. Get ion charges from the periodic table (Group 1 = +1, Group 2 = +2, Group 13 = +3, Group 15 = -3, Group 16 = -2, Group 17 = -1) or from the Selected Polyatomic Ions table (legacy Table E) for polyatomic ions. The periodic table on the Regents reference materials shows common charges in the upper part of many element boxes, so you can read a charge directly rather than memorizing it.

The Criss-Cross Method

1. Write the cation (positive ion) first, then the anion.
2. Write each ion's charge as a number.
3. Criss-cross: the size of each charge becomes the subscript of the other ion.
4. Reduce subscripts to the lowest whole-number ratio.

Example - aluminum oxide. Al is +3, O is -2. Criss-cross gives Al2O3. The charges already give the lowest ratio, so the formula is Al2O3. Check: 2(+3) + 3(-2) = +6 - 6 = 0. Neutral.

Example - magnesium oxide. Mg is +2, O is -2. Criss-cross gives Mg2O2, which reduces to MgO. Always reduce: Mg2O2 would be marked wrong.

Polyatomic Ions and Parentheses

A polyatomic ion is a charged group of atoms acting as one unit (Selected Polyatomic Ions table, legacy Table E), such as nitrate (NO3-), sulfate (SO42-), ammonium (NH4+), hydroxide (OH-), carbonate (CO32-), and phosphate (PO43-). When you need more than one polyatomic ion, enclose it in parentheses before adding the subscript.

• Calcium nitrate: Ca2+ with NO3- needs two nitrates, so Ca(NO3)2.
• Aluminum sulfate: Al3+ and SO42- criss-cross to Al2(SO4)3.
• Ammonium phosphate: NH4+ and PO43- give (NH4)3PO4.

Forgetting parentheses (writing CaNO32) changes the meaning and loses credit.

Naming Ionic Compounds

1. Name the metal (cation) first, unchanged.
2. For a simple nonmetal anion, change the ending to -ide (chlorine becomes chloride, oxygen becomes oxide).
3. For a polyatomic anion, use its name from the Selected Polyatomic Ions table (legacy Table E) unchanged (sulfate, nitrate, hydroxide).
4. For metals with more than one possible charge (most transition metals), use a Roman numeral to show the charge in this compound.

The Roman numeral equals the metal's charge - in FeCl3 each Cl is -1, three give -3, so iron must be +3, hence iron(III).

Naming Molecular (Covalent) Compounds

When two nonmetals combine, do not use charges. Instead use Greek prefixes to count atoms of each element:

• mono = 1, di = 2, tri = 3, tetra = 4, penta = 5, hexa = 6.
• Drop "mono" on the first element (CO is carbon monoxide, not monocarbon monoxide).
• The second element ends in -ide.

Examples: CO2 is carbon dioxide, CO is carbon monoxide, N2O4 is dinitrogen tetroxide, SO3 is sulfur trioxide, P2O5 is diphosphorus pentoxide. The prefix tells you the subscript directly, so prefixes and formulas convert both ways. Notice that carbon monoxide (CO) and carbon dioxide (CO2) are different substances with very different properties - the prefix is the only thing distinguishing their names, which is exactly why molecular nomenclature needs prefixes while ionic naming does not.

The fastest way to tell which naming system to use is to look at the first element. If it is a metal, the compound is ionic - use charges and, if needed, a Roman numeral. If both elements are nonmetals, the compound is molecular - use Greek prefixes. Making this metal-versus-nonmetal decision first prevents most naming errors.

Common Exam Traps

• Not reducing subscripts. Ionic formulas must be the lowest whole-number ratio (MgO, not Mg2O2).
• Dropping parentheses around polyatomic ions when a subscript is needed - Ca(NO3)2 is correct, CaNO32 is not.
• Skipping Roman numerals for transition metals - iron chloride is ambiguous; iron(III) chloride is correct.
• Using prefixes on ionic compounds. Prefixes are for two nonmetals only; NaCl is sodium chloride, never "sodium monochloride."
• Misreading the Selected Polyatomic Ions table (legacy Table E) charges. Phosphate is PO43- (charge -3), not PO4-; copy the charge exactly.
• Forgetting the -ide ending on simple anions (oxide, chloride, sulfide).

Quick Self-Check Procedure

For any ionic formula you write, multiply each ion's charge by its subscript and confirm the total is zero. For any name, identify metal-plus-nonmetal (ionic, use charges) versus two nonmetals (molecular, use prefixes) before applying rules. These two checks catch most errors and turn nomenclature into easy, repeatable points on the exam.