Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 8.3–8.4 Oxidation states and formula writing-Study Notes - New Syllabus

8.3 Indicating Oxidation Number using Roman Numerals

Oxidation numbers can be shown in the name of a compound using Roman numerals. This is especially important for elements that can have more than one oxidation state.

The Roman numeral indicates the oxidation number of the element in that compound.

Where It Is Used

• Mainly for transition metals.
• Also for some non-metals with variable oxidation states.

Format

Name of element (Roman numeral) + compound name

Example: iron(II) chloride

Example 1: \( \mathrm{FeCl_2} \)

• Cl = −1 each → total = −2
• Fe = +2

Name: iron(II) chloride

Example 2: \( \mathrm{FeCl_3} \)

• Cl = −1 each → total = −3
• Fe = +3

Name: iron(III) chloride

Example 3: \( \mathrm{Cu_2O} \)

• O = −2
• Total Cu = +2 → each Cu = +1

Name: copper(I) oxide

Example 4: \( \mathrm{CuO} \)

• O = −2
• Cu = +2

Name: copper(II) oxide

Example 5: \( \mathrm{MnO_2} \)

• O = −2 each → total = −4
• Mn = +4

Name: manganese(IV) oxide

Why Roman Numerals Are Needed

• Some elements have multiple oxidation states.
• Without Roman numerals, names would be ambiguous.

Example:

• Iron chloride could be \( \mathrm{FeCl_2} \) or \( \mathrm{FeCl_3} \).

Therefore, Roman numerals specify the exact oxidation state.

Key Points

• Roman numerals show oxidation number.
• Written in brackets after element name.
• Essential for transition metals.

Therefore, this system ensures clear and unambiguous naming of compounds.