Pick a Cation (Positive Ion) and an Anion (Negative Ion), and the Polyatomic Ion Calculator will work out the balanced Chemical Formula for you — along with the Compound Name, Cation Charge, and Anion Charge so you can see exactly why the formula works out the way it does.
Chemical Formula
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Compound Name
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Cation Charge
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Anion Charge
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An atom has an equal number of protons and electrons, giving it a neutral charge. An ion is an atom or group of atoms that has gained or lost electrons, resulting in a positive (cation) or negative (anion) charge.
To write the formula for an ionic compound, you must balance the positive and negative charges. The total positive charge must equal the total negative charge. Use subscripts to indicate how many of each ion are needed.
A polyatomic ion is a group of atoms that are covalently bonded together and carry an overall electric charge. Examples include sulfate (SO₄²⁻), nitrate (NO₃⁻), and ammonium (NH₄⁺).
Name the cation first, then the anion. For polyatomic ions, use their specific names (like sulfate, nitrate, etc.). If the cation has multiple oxidation states, use Roman numerals to indicate the charge.
Sulfate (SO₄²⁻) has more oxygen atoms than sulfite (SO₃²⁻). The '-ate' ending indicates more oxygen atoms, while the '-ite' ending indicates fewer oxygen atoms in polyatomic ions.
The total positive charge must equal the total negative charge. If the charges don't balance with one of each ion, use subscripts to multiply ions until the charges are equal. For example, Ca²⁺ and Cl⁻ form CaCl₂.
The most important polyatomic ions include nitrate (NO₃⁻), sulfate (SO₄²⁻), carbonate (CO₃²⁻), phosphate (PO₄³⁻), hydroxide (OH⁻), and ammonium (NH₄⁺). These appear frequently in chemical formulas.