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Writing Chemical Formulas for Transition Metal Compounds – A Step‑by‑Step Guide

Transition metals, which occupy groups 3‑12 of the periodic table, can form ions with multiple charges because they lose different numbers of electrons. The name of a compound tells you which charge is present, either by a Roman numeral in the modern IUPAC system or by the traditional "ic"/"ous" suffixes.

Modern (Roman‑Numeral) vs. Traditional (ic/ous) Naming

• Roman numerals indicate the exact oxidation state used in the compound (e.g., iron(III) chloride = Fe³⁺Cl⁻¹).
• ic/ous suffixes describe common oxidation states: "ic" for the most common positive charge and "ous" for one less (e.g., ferric = Fe³⁺, ferrous = Fe²⁺).

Step‑by‑Step Method for Writing the Formula

1. Identify the elemental symbols. Use the periodic table if needed. For polyatomic anions, enclose the entire group in brackets (e.g., SO₄²⁻).
2. Assign ionic charges. Write each ion’s charge as a superscript next to its symbol to aid balancing. These superscripts are only an intermediate step and are not shown in the final formula.
   • Example: iron(III) chloride → Fe³⁺Cl⁻¹ (Fe³⁺Cl⁻¹).
   • Example: iron(III) sulfate → Fe³⁺(SO₄)²⁻.
3. Balance the charges. Convert the superscripts to subscripts so that the total charge equals zero.
   • For FeCl₃: one Fe³⁺ balances with three Cl⁻¹.
   • For Fe₂(SO₄)₃: two Fe³⁺ balance with three SO₄²⁻.

Illustrative Example

What is the formula of cuprous oxide?

• The word "cuprous" indicates Cu⁺¹.
• Oxygen is always O⁻².
• Balancing the charges gives Cu₂O.

By following these steps, you can confidently write accurate chemical formulas for any transition metal compound.