Predict ionic compound chemical formulas from cation and anion charges
The ionic formula calculator predicts the chemical formula of an ionic compound by combining cations and anions in ratios that result in a neutral compound. It uses the cross-multiply method: the charge of the cation becomes the subscript of the anion, and vice versa, then simplifies to lowest terms.
Total Positive Charge + Total Negative Charge = 0
(cation charge × cation count) + (anion charge × anion count) = 0
Cross-Multiply Method
Mm+ + Xn- → MnXm
Then reduce to lowest terms by dividing by GCD
Identify Ions
Ca²⺠(cation) and Cl⻠(anion)
Cross-Multiply Charges
Cation charge (2) → anion subscript: Cl₂
Anion charge (1) → cation subscript: Caâ‚
Simplify (if needed)
Caâ‚Clâ‚‚ → CaClâ‚‚ (1 is omitted)
Verify Neutrality
(1 × +2) + (2 × -1) = +2 - 2 = 0 ✓
Not simplifying subscripts
Caâ‚‚Oâ‚‚ should be simplified to CaO
Wrong parentheses placement
Ca(OH)â‚‚, not CaOHâ‚‚ or Ca(OHâ‚‚)
Forgetting 1:1 ratios
When charges equal, subscripts are 1 (NaCl)
Reversing cation/anion
Always write cation first, then anion
An ionic compound is formed when a metal transfers electrons to a nonmetal, creating cations (+) and anions (-) held together by electrostatic attraction. Examples: NaCl (table salt), CaO (lime), MgSOâ‚„ (Epsom salt).
Main group elements: Group 1 = +1, Group 2 = +2, Group 13 = +3, Group 15 = -3, Group 16 = -2, Group 17 = -1. Transition metals have variable charges (shown in Roman numerals: Fe²⺠= iron(II), Fe³⺠= iron(III)). Learn common polyatomic ions.
The cross-multiply method ensures the compound is electrically neutral (total positive charge = total negative charge). By making the charge of one ion the subscript of the other, we balance the charges. Example: Al³⺠+ O²⻠→ Al₂O₃ (2×3 = 3×2 = 6).
Polyatomic ions are groups of atoms that act as a single ion with a charge. Common examples: NH₄⺠(ammonium), OH⻠(hydroxide), NO₃⻠(nitrate), SO₄²⻠(sulfate), CO₃²⻠(carbonate), PO₄³⻠(phosphate). Use parentheses when subscript > 1: Ca(OH)₂, not CaOH₂.
Name the cation first (metal name), then the anion (nonmetal root + "-ide" or polyatomic ion name). Examples: NaCl = sodium chloride, MgO = magnesium oxide, CaSO₄ = calcium sulfate. For transition metals, include charge in Roman numerals: FeCl₃ = iron(III) chloride.
Always simplify subscripts to lowest whole number ratio by dividing by the greatest common divisor (GCD). Examples: Caâ‚‚Oâ‚‚ → CaO (÷2), Mgâ‚‚Sâ‚‚ → MgS (÷2), Al₃O₃ → AlO (÷3). Exception: Don't simplify within polyatomic ions like SO₄²â».
Ionic compounds don't form discrete molecules; they form crystal lattices. We use "formula unit" to describe the simplest ratio of ions. Example: One formula unit of NaCl contains 1 Na⺠and 1 Clâ», but in the crystal, each ion is surrounded by multiple oppositely charged ions.
Education
Teaching ionic bonding fundamentals
Chemical Formulas
Writing and predicting formulas
Nomenclature
Naming compounds correctly
Lab Work
Preparing ionic compounds