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. 🔬
Exam Tip: Always write the oxidation number in Roman numerals in parentheses after the element symbol.
Example: Fe(III) for iron in the +3 state. ⚡
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. 🎭
| 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) |
Example: In the compound Fe2O3, iron is in the +3 state. Write it as Fe(III)O3.
Exam Tip: When you see a compound like Fe2O3, remember that the iron is not +2 (which would be FeO) but +3. Always check the overall charge balance before deciding the Roman numeral. 📚
| Oxidation State | Roman Numeral |
|---|---|
| +1 | I |
| +2 | II |
| +3 | III |
| +4 | IV |
| +5 | V |
| +6 | VI |
Write the formula for chromium in the +6 oxidation state in a compound with chloride ions.
Answer: Cr(VI)Cl6 (Chromium hexachloride). 💡
Exam Tip: When you’re unsure of the oxidation state, use the rule that the total charge of the compound must be zero. Then, write the Roman numeral accordingly. Good luck! 🍀