Use a Roman numeral to indicate the oxidation number of an element in a compound
Chemical Reactions – Redox
Using Roman Numerals to Indicate Oxidation Numbers
In IGCSE Chemistry, when an element can have more than one oxidation state, we use a Roman numeral in parentheses to show which state is present. Think of it as giving the element a name tag that tells you how many electrons it has given up or gained. 🔬
Why Roman Numerals?
Imagine each oxidation state as a different costume the element can wear. The Roman numeral tells you which costume it is wearing in that particular compound. For example, iron can wear the “+2” costume (Fe(II)) or the “+3” costume (Fe(III)). The costume (oxidation state) is written in Roman numerals to avoid confusion with the element’s atomic number. 🎭
Common Elements and Their Roman Numeral Forms
| Element | Common Oxidation States | Example with Roman Numeral |
|---|---|---|
| Fe | +2, +3 | Fe(III)O3 (Ferric oxide) |
| Cu | +1, +2 | Cu(II)SO4 (Copper(II) sulfate) |
| Mn | +2, +4, +7 | Mn(IV)O2 (Manganese dioxide) |
| Cr | +3, +6 | Cr(III)Cl3 (Chromium(III) chloride) |
Step‑by‑Step: How to Write the Roman Numeral
- Identify the element that can have multiple oxidation states in the compound.
- Determine its actual oxidation state in that compound (use the rules for oxidation numbers).
- Write the element symbol followed by the oxidation number in Roman numerals in parentheses.
- Continue writing the rest of the formula as usual.
Example: In the compound Fe2O3, iron is in the +3 state. Write it as Fe(III)O3.
Quick Reference: Common Roman Numerals
| Oxidation State | Roman Numeral |
|---|---|
| +1 | I |
| +2 | II |
| +3 | III |
| +4 | IV |
| +5 | V |
| +6 | VI |
Practice Problem
Write the formula for chromium in the +6 oxidation state in a compound with chloride ions. Answer: Cr(VI)Cl6 (Chromium hexachloride). 💡
Revision
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